Replacing the 2N3904 with an SCR

So i made a laser alarm system some time back, it worked good but had one fault, more like a limitation. it made the buzzer ''buzz'' only when the laser was interrupted and the buzzer turned off when the laser was allowed to hit the photo sensor again. i wanted it to stay on until turned off by a switch. someone suggested something called a monostable flip flop but i had no idea what it even meant, then i saw something called silicon controlled rectifier or a thyristor, but i also learned that they have to be made using a pnp and npn transistor, what i cant understand is what transistors should i use to replace the npn transistor on my circuit, the tansistor on my cicuit is 2n3904, input voltage is 5v

Question by walee    |  last reply


Where can I find NPN (2N3904)Transistors in scrap/unused electronics, in what devices can they be found in?

I would like to know where I can find an NPN (2N3904)Transistor inside of something I may already have. 

Question by ArtTech86   |  last reply


SMT variant of 2N3904 NPN transistor? Answered

Im looking for a SMT equivalent of a 2n3904 transistor. I found one on digikey that might be similar, the MMBT3904. Is this correct? Many thanks- Astroboy907

Question by astroboy907   |  last reply


joule thief without resistor?? Answered

I want to make a joule thief,  but I don't have the 1k ohm resistor, so what can I use instead? or will it work without the resistor? Thanks for answer!

Question by aninda13   |  last reply


How to add a op amp to this circuit?

I built this circuit a while ago but the only problem i have with it is that i have to use electronics like my iPod, laptop, or anything with a high volume output. So I started reading about op amps and hwo they "jack up the signal" and I stated wondering could I somehow integrate it into my circuit so that my audio input is higher therefore allowing me to use my cellphone, tablet, and e.t.c? Currently I have a LM324 Quad amp i know fr sure i wont need all those pin. I also have a tip31 and 2N3904 transistors.

Question by tramont   |  last reply


How do Transistors handle Amperage?

Hey, I'm working on a wireless power transfer system and I'm planning on running it on 12vdc with a 2n3904 transistor, but my power supply supplies 2 amps. Now the question is, will the transistor draw only the current that my transmitter coils draw, or will it simply draw it all for supplying the coils. I have not yet determined how many turns or what resistance they have yet, I'm still in early stages. I just don't want to ruin one of my few transistors. I know the absolute maximum current this specific transistor can handle is 200ma.

Topic by DecemberRain01   |  last reply


How do I know if a transistor is a 2N4401 or a 2N3904, etc? Answered

Good day. I'm a newbie scavenger, and I scavenge parts since I don't have enough money to buy new parts. I want to know if the transistor i found is a 2N4401 or a 2N3904 or another? I can't find it in the Internet. One thing I know is that it's an NPN transistor. I also noticed some numbers on the flat side of the transistor. Does it help in determining the transistor? Thanks in advance to those who will answer. :D

Question by CarlGemarino   |  last reply



help making a amplfyer? Answered

I was going to build a amplfyer from this instructable but had no BC548 or 2N3904 transistors. I did a google search to find out what could also work. I found a website stating simmalar transistors. On this website, at the related transistors section it states a few simmalar transistors. What transistors could I use instead of the BC548 or the 2N3904?

Question by David97   |  last reply


How do I use MOSFET's for switching? Answered

I've been confused about this for a while now. I know how to use bipolar transistors(2N3904), but not MOSFET's. I've got one of these.

Question by geeklord   |  last reply


Need help with JFET preamp and chossing transistor ? Answered

Hello .I'm having some some problems with my current project .Can you guys help me please ! This is the schematic of MXR headphone amp for guitar .I don't have 2n4401 and 2n4403 , so i replace them with 2n3904 and 2n3906.The result is output is too low .2n4401 and 2n4403 (collector current = 600mA) ,2n3904 and 2n3906 (collector current = 200mA).If i replace the transistor with higher Ic i will have louder output ??? http://i247.photobucket.com/albums/gg132/chipmapple/mxrheadphoneampschematic.gif Thanks for advance! Sorry for my bad English!

Question by login721   |  last reply


What transistors have a very low switching current? Answered

I need a transistor that "switches" when I apply around 0.5 volts/1-10 milliamps to its base? 2N3904 transistors require too high of a switching current(is that what it's called?). Does anyone have suggestions?

Question by seraine   |  last reply


How can I replace a MOSFET with a common transistor? Answered

This is a flip-flop circuit that I want to use for a project. It calls for any N-Channel MOSFET, but I don't have one. Is it possible to replace it with a transistor, such as a 2N3904?

Question by 1up   |  last reply


Transistor Related Questions?

Hey Guys, I need some help, I need to choose a transistor and get to know how to wire it. I have the following transistors: 6822 (http://pdf1.alldatasheet.com/datasheet-pdf/view/604250/ALPHAWIRE/6822.html) 2N3904 (https://www.sparkfun.com/datasheets/Components/2N3904.pdf) 19001 2N2222A (http://www.onsemi.com/pub_link/Collateral/2N2222A-D.PDF) TIP42C (https://www.fairchildsemi.com/datasheets/TI/TIP42.pdf) And I need help to choose one that can run an Green LED Strip (16 in.). And also how to wire it. If you can give me the steps to wire it up to the output of a circuit and how to wire it up with the led strip it will be greatly appreciated. Oh and also the voltage at which the transistor turns on should be 2.8-9v ish. Thanks in advance. -Prickly Potato

Question by SA-DIY   |  last reply


I want to make a joule thief to power a 10W LED (833ma @ 12v) from ~2 AA batteries, suggestions on toroid ? # of turns ?

My transistor inventory: Part #          Package TIP122       TO-220 TIP32          TO-220 2N2222      SMD 2N3904      TO-92 2N3906      TO-92 C1815        TO-92 BC546        TO-92

Question by yaly   |  last reply


My joule thief is not very bright? Answered

My joule thief is not very bright? I've changed the windings, transistor and the resistor; and it is still pretty dim. Help please Parts: 2 different size toroids: Large and Small 3 Transistors: 2n2222, 2n3904, 2n5830 2 Resistors: 1k, 2.2ohm blue led white led with a heatsink Also I used a 20awg and 36awg to make the toroids

Question by daemonral   |  last reply


Newbie Alert!Is it possible to make a 8x8 led matrix with 74hc595 shift registers w/ arduino?

If so can someone help me with the schematic? And also I'm using an arduino uno r3 if that helps? I kind of would need help with the programming as well. I have transistors (2N3904) and resistors (100 Ohm/200 Ohm) too.  Thanks for the help!

Question by scooter2013   |  last reply


what i can use to replace MPF102 ? Answered

Hello , i making the little gem mk2 (http://www.runoffgroove.com/littlegem.html) the problem is i cant find any jfet mpf102.I have some other transistor, witch i can use to replace the mpf102 ? 2n3904 2n3906 2sa1015 2sb564 2sc1815 2sc2222 2sc945 s8050 s8550 s9013 s9014 s9018  thanks!

Question by login721   |  last reply


pwm arduino

I have a couple questions about driving motors from the arduino (I have a boarduino) - First off, these motors are from various walkmans and I think each one is rated for 1.7 volts. - If I need a transistor, can i use a 2n3904 (since these arn't giant motors) - How do I protect against the emf? The motors need to work going either way (clockwise or counter clockwise) - How would you program it so that the motor can go either forward or backwards?

Topic by guyfrom7up   |  last reply


Joule thief is not working?

Learned about the Joule Thief from Make's Magazine and used their schematic. I'm pretty new to electronics so I don't know much about components. Applying 3 V, the LED lights up, but applying 1.5 V, no light. Green wire is 22 ga, blue wire is 26 ga. Using a 2N3904 Transistor and a Ferrite Toroid. I think I "barely" got the LED to dimly light up, but it would not stay on. Am I doing something wrong? Picture available in second comment.

Question by gera229   |  last reply


I have a large bipolar stepper motor. What's the simplest circuit to drive it with a microcontroller (arduino)?

I have a large stepper motor with 4 wires (bipolar) from an old printer and I assume it's 12 volts, or at least higher than 5v. I have plenty of transistors: 2n3904, 2n2222, 2n4401 and 2n3906 (PNP). How do I drive the stepper with output from an arduino? (simple H-bridge schematic?) Also, how can i find how many steps it has? Thanks!

Question by Vick Jr   |  last reply


Does any one know how to make an Ne-2 joule thief ??

Recently I’ve been researching on how high voltage joule thief works. The popular high voltage joule thief videos out there are known as Jeanna’s light if I’m not mistaken. I also built my own little LED joule thief circuit, it appear success after all with a 2n3904. But I want to take the concept a little bit further, so I decided to attempt on the high voltage joule thief circuit. It appear no luck, I have 120 turns for high voltage on my ferrite core (1 turn = 1 volt). I know it’s the same principle as the LED joule thief but with the LED removed. I have two 1kohm resistors and I connected them in parallel to get 500ohm resistance. I connected them all like the LED joule thief, but it seems like I did something wrong.  So I ask for further instruction. Parts that I use: 40 or 30 gauge magnet wire (high voltage windings) 20 gauge magnet wire 2n3904 transistor (Don’t know if that’s the right transistor for this circuit, but it’s for experimental purposes only.) 2x 1Kohm resistors in parallel = 500 Ohm resistance AA rechargeable battery. The thick magnet wire are 7 turns.

Question by BlackWolf1024   |  last reply


How to make Joule Thief with Multiple LED and maintaining its brightness.

I've seen people running 61 LED or 100 LED on Joule Thief (1 AA Batt) with its full brightness. How did they do it?? I know how to build Joule Thief I have: -NPN   2N3904, 2N2222A, 2N4401   Transistors -Resistor   what I have now 1K, 560, 100.. -Toroid 12 turns and 20 turns -LEDs! -Batteries! Can anyone tell me what to do to run at least 10 LEDs with good or full brightness?

Question by jaesungauzakim   |  last reply


Turn 2 regular dc motors on and off with arduino safely?

I have 2 regular old DC motors (one like this and one like this) that i need to turn on and off at the same time with an arduino. (No direction or speed control, just both of them full on on or full off.) I have a hacked computer PSU to power them and the arduino . I have a bunch of 2n3904, 2n3906, and 2n222 transistors,a bunch of 1N914 diodes, a bunch of resistors, and some other random parts. What other parts do i need and how do i do this easily, reliably, and safely? Thank you!

Question by Vick Jr   |  last reply


Witch transistor should i choose for low noise audio ? Answered

Hello , im making a guitar pedal .The original schematic use transistor BC107B (epitaxial NPN) but i dont have it.so i find some other NPN transistor to replace it. I have 2N3903 ,2SC1815,2N3904 , but witch should i choose for low noise application ? in their datasheet i found the "NF : noise figure" , the lower NF mean lower noise ?? BC107B         NF  Typ.= 2  Max.= 10 2N3903          NF  Typ.= -  Max.= 6 2N3904          NF  Typ.= -  Max.= 5 2SC1815       NF  Typ.= 1  Max.= 10 Thanks for advance! sorry for my bad English.

Question by login721   |  last reply


What value resistor do I need for a transistor base? Answered

Hello, I am trying to work out what resistor I need to use on the base of a 2n3904 base. I am using a 555 timer to flash some leds all powered by a 12v power supply. There are 4 leds in series each with a forward voltage of 2.1v and they require 50ma. I have found that I need a 82 ohm resistor to protect the leds using a online calculator. Pin 3 is driving the transistor base and the transistor controls the leds but I do not know what resistor I need on the base. Any help is appreciated! Edit: here is the finished circuit. http://vimeo.com/33513619

Question by Alex1M6   |  last reply


Can I get help making a simple circuit for two LEDs?

I can't find a circuit online or get any of my experimental circuits to work. I have been trying circuits with transistors - specifically the 2n3904. All I want to do is make a circuit that has two LEDs and any kind of switch. I want one LED to always be on. Then, once the button is pressed, the first one goes off and the second one comes on. I do not want to use an arduino or anything like that. I don't really care what components are needed I just want it to work. Any help or guidance is appreciated. Thanks

Question by knex.gun.builder   |  last reply


What do i need to control a sprinkler valve (24V solenoid) with an arduino?

I need to know what parts to get to open and close an electric sprinkler valve with my arduino. It's supposed to use 24V but I've heard that 9V works just as well. I tried using a 2n3904 as a switch with a 9V battery and it turns on but won't turn off unless I disconnect it. Since it's a solonoid (inductive load), I assume I'll need some sort of protection diode and noise-canceling capacitor. Would a MOSFET be best to control it with 5v output from an arduino? What models for these components would be best (and can be bought at radio shack)? Thank you!

Question by Vick Jr   |  last reply


Circuit not working, transistor heating up?

Hey guys, I made this circuit (diagram attached) but it isn't working. There is no output voltage and the transistor is heating up like crazy. Some differences in my circuit: 1- I got my transformer from an LED bulb and not a camera. 2-I'm feeding it 3v for testing. 3-The NPN transistor I used is 2N3904. 4-Im using 2 100 ohm resistors in series instead of one 220 ohm. 5-I didnt attach a potentiometer, 6-I couldnt find a capacitor like that so I used a polyster 400v 473J capacitor. 7-My diode is not a schottkey one but a 1N4007 diode. I double checked al my connections and there are no shorts.

Question by SA-DIY   |  last reply


switching on a device using a mcu

I just want to know, I have a device (a gravis game pad pro) that continuously sends out a 3 byte serial packet via 2 wires (the game pad drives the clock, so the mcu is the slave, the clock is at 20khz), but there isn't any chip select or slave select or anything like that to start the serial transmit, and since my application circuit already has another serial device on connected to the spi pins of my PIC, I can't have the game pad on all the time, so I decided I need to control when to power on the game padwhat should be used (that i have on hand right now)?a switching regulator like the TC110 from microchip? (i don't have this one yet)a mosfet driver (i have lots of tc4424)?2n3904 or 2n2222 or bc547 transistor?EDIT: I'm just gonna use the mosfet driver... But what is the proper way to do this?or waste some program space and manually interpret the clock and the data pins? there is always five consecutive "1" in the first byte of data, and that condition can never be repeated

Topic by frank26080115 


Transistor NAND Gate does not completely work.?

I have been trying out some logic gates using transistors and I am confused already. I have built a NAND gate using 3 2n3904 BJT's. The resistor values are aprox the values in the schematic. The Diodes are 1N4003. Vcc is 5v (~4.9v) The schematic can be found here https://www.instructables.com/id/Digital-Logic-Gates-Just-Using-Transistors/ I have everything hooked up like the schematic shows. The issue I am having is that it is not working as I would expect a nand gate to work. The truth table shows that if A and B are both Low then C is High.  If A is High and B is Low then C is High. If A and B are both High then C is Low Here is what really happens.  A Low: B Low: C High A Low: B High : C High A High: B Low : C Low (but still ON just very very dim) <=Shouldn't this still be a High state? A High: B High: C Low Does any one have any ideas as to why it is behaving like this. I have rebuilt this circuit about a dozen times now making sure I have things correct. I am stumped.

Question by atrumblood   |  last reply


How can I boost current to multiple IR LEDs

Hi, I've been working on a project with an arduino to use it as a TV-B-gone, now i know this has already been done and I have found lots of helpful documentation and code so far. I've wired up a matrix of LEDs in a 4x5 pattern exactly like this one https://www.instructables.com/image/FRG2JXC3ALEP282OH6/Complete-the-circuit.jpg taken from this project https://www.instructables.com/id/Ultra-TV-B-Gone/ . The design works, but with the setup I have now, i'm just supplying whatever current the arduino is outputting to the LEDs, I know I need some resistors and a transistor and thats what I need help with. I did also buy a 2N3904 transistor like the one in the project however, when I wire it up, nothing works. The lowest resistor I have is 220 ohms and I've tried wiring that up in various places throughout the circuit. Does anyone have any tips that would help me maximize the range for this LED matrix? thanks, ill be taking circuit classes next semester so one day ill be able to figure this out myself.

Topic by jcouch1227   |  last reply


Troubles making a BEAM bot (Miller engine)

Hey everybody! I'm just getting involved with the wonderful BEAM bots. The problem is that here in Germany I don't get some of the transistors and I have to find alternatives. Instead of the 2N3904 I use the BC546, for the 2N3906 I use the BC556 and for the MN1381 I use the TC-54. I'm attaching the datasheets - did I get it right that I only have to "turn the BC556 around" so the pins fit? However, I don't get the Miller engine working! As a solar cell I use a Panasonic BP-376634 (3,4V - 30mA) http://shop.aaaa-electronic.de/epages/15164820.sf/de_DE/?ObjectPath=/Shops/15164820/Products/50007-002 As capacitors I used different ones. The motor is a tiny cell-phone vibrating motor - I guess one of the tiniest. As a resistor I'm using a 2.2K. All in all I tried to follow these diagrams: http://www.solarbotics.net/library/circuits/se_t1_1381.html I also uploaded some photos - maybe you can make out mistakes I made... Hoping for your help! Sebastian

Topic by sebarr   |  last reply


What to buy if your a beginner

Hello, I'm 13 and I want to get started in electronics. I know how to solder and read basic schematics and I have two of those 200 in 1 electronics sets. I am about to make an order for some components from www.bitsbox.co.uk.This is what I have so far, anything you would add or remove? (I couldn't easily get the quantities)pp3 battery clips2 x AA Battery Holder3 x AA Battery Holder11 wire single core bundle1uF 50V Electrolytic Radial100uF 25V Electrolytic Radial220uF 16V Electrolytic Radial22uF 50V Electrolytic Radial25-way PCB D-connector PlugPrototyping breadboard 80x60Watch crystal1N4001 Rectifier Diode 1A 50VStripboard 95x64White LED 5mm 30000mcd24V 16A SPST K4TA24 RelayPiezo Transducer with Flying Leads7805 voltage regulator (to get a steady 5v from a wall wart for the breadboard)NE555 timer IC2N3904 NPN transistorBC213 TransistorATTiny2313-20PUlm386 op-ampStandard Toggle SPSTupdates:I have a multimeter, resistors and LEDs and I am buildng an attiny 2313 programmer as well as adding to my collection.

Topic by josh92176   |  last reply


How can I multiply/double (or just increase) voltage in a DV 5v curcuit ???

ok i have the following parts to ue for this: (all leftovers from former project..)  :) 1x mosfet IRF510 power transistor 5,6A 100V 9x 2N3904 amplifier transistors 40V 0,2 A .0.625 W ~16x 1uF capacitors 50V ~99x recifier diodes 1A 50V ~35x 2k resistors  0.25 W the idea is to charge a battery pack consisting of 4x 1,2-1,25V = 4,8-5V 3000mah AA nimh batteries using my 5v usb wall charger that has 1A output! i tried connecting the battery pack directly to + and - and 'i think' its starting to charge but after 1-2 hours at full rate (5v 1a) it should be at least 50% charged (3 hours would be full charge -> 5W x 3 = 15W), but if i connect them they last for about 2 -3 minutes and then empty again!! so after this im concerned that i might need at lest 6V-10V to achieve the desired effect !!  -> charge 15W NIMH batterpack with wall charger (nokia ac-60e.. 1-1,5A) i nless than 3 hours (as like a phone would need !!) therefore i need a cuirciut (ONLY DC) to double my 5V OUTPUT to 10V (yiou shouldnt apply more than twice the curent of a batter pack) or ideally 6-7 V but i dont know if thats possible without a swiching IC or more different resistors.. so just doubling would be nice at the moment!!  

Topic by sblocc10   |  last reply


Arduino-Based Solar Heater for My Above Ground Pool

A few weeks ago I grabbed the garden hose to top off the pool and noticed how hot the water was as it came out. (the hose had been lying in the sun). So, I decided to put it to good use. We've been looking at solar heaters for the pool but found the prices prohibitive, and being a DIY'er I decided to build my own. I have 50' of black, tangle-free hose coiled on a 2' by 4' piece of Wolmanized, 3/4" plywood. This is perched on top of an 8' 4by4 post which I sunk into the ground near the pool's filter. To feed the hose I installed a faucet in the filter's return line which is under sufficient pressure to travel up and into the hose. The other end comes down to an electric valve before emptying back into the pool. The Arduino portion of the project consists of an Arduino Uno board, a DS18B20 temperature sensor probe, a 12-volt, 3-amp power supply, A small prototype board that holds two 12-volt relays and two 2N3904 transistors which the Arduino triggers, two 12-volt car-type relays which are triggered by the smaller ones, and a Mallory Sonalert. My idea was to have the Arduino monitor the temperature in the return line from the solar hose coil, and when it reaches 80 degrees F, the Sonalert goes off for 5 seconds to warn swimmers that hot water will be coming out of the return spout in the pool. And after the warning sounds, the electric valve then opens for 2 minutes, flushing the warmed water into the pool. Since the system is driven by the pool's filter, the solar hose is replenished during this flush period, and when the valve closes, that new water is then heated, and the whole process starts over. The Arduino code consists of a couple libraries to incorporate and read the temp' probe's output, a subroutine which tests both the Sonalert and the valve when the system is first plugged in, and then the necessary code in the Void Loop section which simply waits for the temp' to reach a preset level and then sound the alert and then open the valve.The system is built into an 8" by 6" electrical box that mounts on the 4 by 4 post just under the solar hose panel.The systems been up and running for about a week now and I've averaged about a 2 degree rise in the pool's water temp' per day.If anyone is interested in seeing photos of the project, or the Arduino code, I'll be happy to provide. UPDATE: Here are some phots of the project...

Topic by bnelson218   |  last reply


P-Channel MOSFET woes - anyone know how to drive one of these things with Arduino?

Hi all, I’m a hobbyist trying to build a digital controller / power monitor for a green power generator project built from an auto alternator. Hoping to use an arduino microprocessor & some simple transistor circuits to power an alternator’s field coil which at 13v can draw up to 10amps of power. From what I’ve read I think I need high side PWM switching because the alternator shares a common ground with its other electrical connections preventing me from using my usual novice NPN based bag-of-tricks. From my reading I assume some sort of PNP or P-Channel MOSFET is in order (components I sadly have no prior experience with). Alternativly I've read about using fancy high side controller ICs to drive regular NPNs but don't want to go that route. So far everything seems to work except I clearly can’t figure this P-channel MOSFET out (IRF6540)!  No matter what I try I always get a high voltage at the drain pin (full-on condition) regardless of what I do to the gate pin. I uploaded a portion of my schematic as a test circuit I think should work from my limited understanding and online tutorials: http://www.curtismasonjohnson.com/sandbox/ProblemCircuit.JPG   - The NPN driver in the drawing (2N3904) by itself works when tested with a voltmeter and toggling the input   - If I replace the p-channel mosfet with a PNP transistor it work as expected. Shouldn’t this mosfet work pretty much the same? My puny TIP127 PNP can't handle the current if I were to hook up the load.   - I’ve tried several IRF6540 components   - I’ve tried connecting the MOSFET’s ground pin via a current limiting resistor directly to ground. Also via the resistor to 13v. No voltage change on the drain pin in either case. Clearly I’m missing something embarrassing and fundamental. I mentioned I’m a hobbiest, right - my day job is managing global marketing programs for fortune 500s and don't know anyone that knows anything about electronics! After years of electronics & robotics projects I’ve yet to be stumped to the point of needing help til now - amazing how much you can self-teach via google and such. This one has me beat – hope someone can help as I have no idea what to do next! Truely appreciated...

Question by cmjstealth   |  last reply