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Is this electronics schematic correct? Answered

Hello, I have drawn this simple schematic and I am wondering if someone could look at it and see if it will work. The thing I am not sure about really is if the transistors are connected in a way that will make them do what I want them to.

This circuit is designed to be off, then come on and stay on with a single push of the momentary switch, then turn off and stay off if something blocks the light to the photo-transistor for an instant.

****** I have updated the schematic based on the comments I got. The decision was that the circuit would come on by itself without a push of the momentary switch. The new schematic is the first picture, the old schematics are the second and third pictures. I have a breadboard and some parts for the project this circuit will be used on coming in the mail, so I can start trying this out when it gets here. But for now I am still not sure if this is right, I don't have much experience with transistors. Any advice is appreciated.



Best Answer 4 years ago

the link between the E pin of the first transistor and B pin of the second transistor means when the first transistor is on the second will always be on as it bypasses the switch. which means as long as there is light, power will flow through the load.you need a circuit a bit like the one shown in this video http://www.youtube.com/watch?v=I70H5xQ6MT0 Hope this is a little help.

Yes that is helpful thanks.

You said "when the first transistor is on the second will always be on as it bypasses the switch." Yes I mean for it to do that, once current starts flowing is should stay on until the phototransistor turns off.

The other part of the idea is that the first transistor should not be able to come on unless the second transistor is turned on by the momentary switch, because there is no access to the - side of the battery for the first transistor unless the second transistor is on. UNLESS current is able to flow from E of the first transistor, through B-E on the second transistor. That must be the problem you saw?

yes it was. your new schematic looks a lot better. playing around with components will help you work out what works and what doesn't, a lot better than theory so i would give it a try.

I also found this:

I think the relay in that circuit could be swapped out for a JFET and an NPN transistor working together (one as the normally off pole and one as the normally on pole). I need this switching to be super fast that is why i wouldn't use a relay for this.

bad schematic but t1 should be a pnp not a npn

That would fix it? I do not understand, what will that change?

Thank you very much for your help. It looks like your design and my second design require the same parts (the BJTs you mentioned are the same ones I had already purchased!). Yours offers the advantage that the LED is not always on, and the main current only has to flow through one transistor instead of two.

I stared at both circuits for about 10 minutes thinking and suddenly realized a fatal disadvantage of the design you have offered though (nothing you could possibly have known about, it is specific to the mechanism this will be mounted to). There will be several of these circuits being used very near each other and light could easily transfer from one LED to the wrong photo transistor and activate a circuit that needs to be off. On the second design I have drawn, it can only be turned on with a signal from both the phototransistor AND the momentary switch.

Use a shield between the circuits if one will activate the other.

Mechanically blocking the lights from each other is not possible in this case.

Yes, it's just that the way the lights and sensors need to be positioned they will be in view of each other. There is no way to block it off without interfering with the function of the mechanism.

I agree that your circuit has several advantages but I need a circuit where the phototransistor is not able to activate the whole thing without another separate signal.

What I don't understand is how is your circuit going to avoid the same malfunction.

Sorry for taking so long getting back I'm cleaning electronics gold.

On the newer schematic I drew (the upper picture) the signal to the "base" pin of each transistor comes from the section of wire between the two transistors. So if the transistors are both off then there will be no connection from the battery to either base pin. The phototransistor in my circuit can interrupt the signal to the base of one of the transistors to shut it off, but does not connect the base pin to the battery unless current is already flowing in the wire that is between the two transistors (meaning the momentary switch has been pushed).

The thing about your top schematic is as long as the LED is on the Phototransistor is closed and the base and emitter of the PNP transistor act like a diode (inside the green box) making the circuit permanently on as current will follow the red arrows closing the circuit the blue arrows.

Oh I see... It seems like this should be a lot easier to figure out. I need more experience with this.

Well, one solution could be to put the LED in parallel with the load like it is on your circuit. That I am hoping finally solves this... I will change the image of the schematic again for that correction.

Oh yeah then a neighboring light will turn it on. I will have to think of something else.

You need to shield whether a circuit closer to yours or mine you need to shield light is fickled. Your circuit will shutoff depending on transistor tratis and if it is active with or without load. Not an easy circuit that is why icing said clever circuit it is minimal.
If I knew what you are trying to do I could be more helpful.

Thank you for your help and advice. This is a strange puzzle.

I may have just come up with one that will work. My approach this time was to make a circuit that says "phototransistor must be on for current to flow," and a separate circuit that says "circuit will not turn on by itself, requires outside signal, but can stay on once going." Then connect those two circuits, the load, and the battery all in series with each other. LED can be in parallel with load so it is only on when it needs to be and lasts longer / uses less power. I have made the new circuit the first picture.

This circuit will be part of the timing circuit for a coil gun. It will use an unconventional coil timing. There is a phototansistor and LED between each coil. 3 coils come on at a time. As the projectile passes a phototransistor (blocking the light), the coil behind that phototransistor turns off and the next coil out in front of the set of 3 turns on. This way a wave of 3 "on" coils moves down the accelerator keeping the projectile somewhere in the first coil. The last coil shuts off ahead of the projectile. When I have it all working I will share the designs in case anyone else wants a similar circuit. No reason to do that until it works though.

I am building a coil gun also and I plan to fire the coils the same way as you however I plan to use rotating EM waves based in Fibonacci sequencing. The pic is of my 3D printed coil core and barrel.

Out of the magazine my projectile will be traveling 250 fps first stage 250 fps, second stage 500fps, third stage 750 fps, and fourth stage 1250 fps, or the same speed as a 22 caliber bullet.

This is my trigger circuit only instead of firing coils it will trigger four three stage Johnson counters that will fire the coils in each stage.


BarrelAssembly.jpgAnswer 3.png

Wow that is an incredible speed for a homemade coilgun! Those 3d printed parts look great. I can't tell from the picture, about how long is your barrel? So your accelerator is partly optically timed (one optical trigger every third stage) and partly passively timed by "Fibonacci sequencing"?

I originally wanted to use a passive coil timing but decided it would be to difficult to get it right, and that it would only be timed correctly as long as you always use the same projectile and the power supply is always at full power. The ideal coil timing for a "reluctance" type coilgun in my mind would be one with hundreds or thousands of stages, each the width of on strand of the coil wire. They would be passively timed to keep the projectile at the ideal point in a coil of ideal dimensions and maximum power based on the magnetic saturation point of the projectile (from what I understand, above a certain point you can not increase the force on a ferromagnetic projectile by increasing the magnetic field strength).

I will give a little more detail on my coilgun to clarify. It will have no capacitors and charging circuits, power will come directly from lithium batteries. It will be selectable semi/full auto, that is part of why there will be no capacitors. The accelerator is 3ft long 19 stages. Each coil is about 1.75" long, 5 layers of 14 AWG magnet wire. For the projectile, I will try 7/32 steel balls (they fit the barrel perfectly) and probably cut some other projectiles from steel rod. The advantages of balls are that it doesn't matter if they rotate randomly in flight (there is no rifling), they will be easy to load and store in a magazine, and they don't need to be cut by hand (they come pre-made).

Yes part optical part Fibonacci 1 1 2 3 5 think of it a title wave stronger and faster as it goes forward.

What is giving me a hard time is the clock for the Johnson clock, it’s the three timer, one, two, and five are easy it is the three that is buggering me. I can do it with a PIC but the EMP of the coil gun buggers the PIC, and I haven’t made an EMP shield good enough to stop it.

On the 3D printed parts the stages are 3 x 1 x 1 inch 5 stages for the barrel and action making it 15 inches. The ball barring magazine is 8 ¼ inches long it will fire 5.5mm or 22 caliber ball barring.

I am planning on powering the coils the same way as you constant power during firing and incase the EM wave is out of sync with the projectile it will re-sync when it passes the phototransistor.

The gaps between the sections and the holes in the action is for the LEDs, phototransistors, and air. In the first couple coil guns I built I found you need to dissipate the plow wave in front of the projectile and the vacuum behind the projectile or they will slow down your projectile as it tries to move through the barrel.

If you are making a 19 stage you will need air out and in to the barrel, as for my trigger just add more center circuits remember the circuit on the clear is at the end of the barrel so the coil gun is ready for the next shot and my circuit is based on positive logic.

Yes there is positive logic and negative logic.

I went out to my lab and dug these up, I get these from scanners and printers and they are ideal for coil guns, they are LED, phototransistor, and shielding all in one unit. They sense the passing object, in the case of the printer paper and tell the computer. Which is what we want for the coil gun, I love to work with salvaged parts.

By the way did you see my Instructable on magnetic wire:



That idea of using the optical sensors only every few stages to re-sync sounds like a good plan. I don't know if I can help you with your clock, other than offering ideas about emp shielding and suggesting another type of timer.

One other type of timer that I almost used is a mechanical one. You could have several copper contacts arranged in a straight line, spaced with linearly increasing distance between them. The exact spacing can be adjusted by screws, with a sort of linear worm gear.
A spring propelled sliding contact can slide along and make a brief connection with each of the fixed contact points. That can produce the timing for the main power switches for each coil.

For emp shielding, you can put the sensitive electronics in a completely enclosed aluminum container with small holes for the wires. All wires coming out can be wrapped around ferrite rings.

I had not seen that instructable before. It looks like it could be helpful when looking for this stuff. I have taken in my share of TVs from the woods for parts lol. Fortunately I was able to get 19 pounds of that 14 AWG magnet wire for $102 after shipping. Still expensive but for that wire it was a good deal I think.

I must run but with these circuits you will only need one LED and one photo transistor between your coils. I will explain in a couple hours.

Answer 3.png

Nice and simple isn’t it.

The momentary switch turns on the LED and load and it stays on until you block the light from the LED.

OK I have changed the design based on your suggestion. Here is how I think it should work:
1-Both transistors are off, no current is flowing.
2-The momentary switch is pushed, and the NPN transistor turns itself on. That allows the PNP transistor to turn itself on.
3-The current should continue unless the light is blocked from the photo-transistor.
4-When light is blocked to the photo-transistor even just for an instant, the PNP transistor should turn off and circuit should stay off until the momentary switch is pushed again.

What are the connections to the transistors ? You have misidentified the collector and emitters.

Oh sorry I didn't realize it was backwards on a PNP transistor. As I have mentioned don't have much experience with transistors yet.

Its not. The pins are the same names and shape, only the arrow points into the base on a PNP. I ask, because I don't know which is what you think you want.

It was backwards from what I thought it was on the symbol. I had thought whichever one holes flowed out of was the emitter. But after looking at a couple pictures it looks like holes flow in the pin that is considered the emitter on a PNP and holes flow out of the emitter on an NPN.

Its the direction of flow of the MAJORITY carriers, in a PNP its "holes" in an "NPN" its electrons.


4 years ago

What voltage and current is your circuit ?
Are you concerned about limiting base current to the transistor ?

I include a pic of transistor action.
Which is as true for TO-92  and   PNP.


The voltage and current is kind of irrelevant, I will put in resistors as necessary to produce the right voltage for the transistors. The purpose of this circuit is to produce the correct on-off signal for some power transistors.