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Timer Circuit

I found this simple circiut on jamesyawn.com, and was wondering if and how to change it so that it could time in the minutes, longer than it does already. I don't know anything about electronics, aside from soldering, and basic connections, so try to give a simple-ish answer. I also wondered about what transistor to use (like a radioshack part number). The circuit I was talking about is under "discrete timers".

(update, more questions)
What kind of timing (like how many seconds) would I get with this?
Using the same stuff shown on the schematic?

Also, how do I pick a transistor?
Are they interchangeable?
What specs should I be looking for?

If this seems like too much, could someone give me a link to a site that would be willing to help with (very) beginning electronics?

AND, how would you recommend making the transfer schematics for home-etched circuit boards? I can't get the component spacing right.

Picture of Timer Circuit
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AndresV31 year ago

maybe if you change the resistor from 100k to 150 or 200 k it may increase the tau and get more time, but this may change the circuit properties so give it a try.

You do realise the topic was dead for 5 years until you ressurected it? LOL

555 timer is the next step, look for it on the internet. Radioshack sells it. Its simple to use and I got up to 30min with it. But its not exact its more in between 25-35min.(it changes). You want the astable, or osillator mode when you research it, that will make more sense when you look for it.
Austringer8 years ago
A while back somebody offered me a heap of ego with very precisely zero advice on one of the Make comment threads. I'm going to give you the advise he would have given me had he not been all about letting me know how smart he was. Track down a copy of "The Art of Electronics" by Horowitz and Hill! It's a text book and, ergo, not exactly cheap, but I'm finding it worth it. To answer one of your questions up there, a capacitor charges in roughly 5RC where R is the resistance in ohms and C is the capacitance in Farads. In the diagram you have your looking at 150,000 ohms x .0002 Farads so 2. 5 minutes to FULLY charge the capacitor. Your transistor will do it's thing before that happens. What are you wanting to do with this? How exact does it need to be?
gmoon9 years ago
This uses a PNP transistor. Radio Shack sells the 2N3904, a PNP--but it can only handle 100mA (I've seen small relays that only draw 30-40mA, but an automotive relay is anywhere between 80-140mA....)

I'd use a 2N2222--cheap, easy to find, and can sink 1A. But they are NPN transistors, so change the circuit: Transistor circuits link.

You might consider using a 555 instead of the simple RC circuit....

Pretty old project there... Ironically, it's described as " Time delay relays are basic electronic circuits which serve to protect costly equip. from damage due to current surges ", but there's no surge-protection diode on the relay! Check out the link above for an explanation of the diode placement.
John Smith (author)  gmoon9 years ago
Ok, I'm lost. I need to find a simple electronics book.

Anyway, would a 2N3055 work? It is NPN, and I have one in the garage.
The specs of it are here. (at the end of the NPN section)

Thanks for the help.
I don't see why not...but that's serious overkill for a relay! (15 amp rating !!) It's gain (Hfe) is rather small, so it will draw more base current (no biggie.)

Most of the example on that link are NPN, so start there. As far as converting your original schematic (PNP), it's essentially upside-down compared to the PNP circuit on the 'Transistor Circuits' page.

I.E., (for your schem) the power source polarity-- the GND (-) is at the top, and the positive is at the bottom...Use the NPN circuits, and convert the 'timer' part by flipping it top-to-bottom...

Both Goodhart and I made the point about spike protection, so pay attention...;-)
John Smith (author)  gmoon9 years ago
GAH! I'm all confused!

...As far as converting your original schematic (PNP), it's essentially upside-down compared to the PNP circuit on the 'Transistor Circuits' page.
What do you mean "upside down"? To what PNP circuit on that page? I didn't see a timer there.

I.E., (for your schem) the power source polarity-- the GND (-) is at the top, and the positive is at the bottom...Use the NPN circuits, and convert the 'timer' part by flipping it top-to-bottom...
What NPN circuit?

Both Goodhart and I made the point about spike protection, so pay attention...;-)
*sigh* I guess now I have to ask what the heck you are talking about...
SO, what is "spike protection"?

I am SO sorry that I am making you do all of the work, I just don't understand this stuff. I've tried, but the second I get to understanding it, I get confused again!


The "that page" link reference...

1) What do you mean "upside down"? To what PNP circuit on that page? I didn't see a timer there.

--There is only one PNP circuit on "that page." Since you want to use an NPN instead, you'll need to convert your PNP circuit to the way they draw them on "that page..." The power source polarity on their drawing is upside-down compared to yours....

--Yep, no timer there, that's why you want to convert your schematic.

2) What NPN circuit?

--Any NPN circuit on "that page"--might as well get a working transistor-switched relay before tackling the timer...

3) SO, what is "spike protection"?

See the Protection diode section on "that page" :

If the load is a motor, relay or solenoid (or any other device with a coil) a diode must be connected across the load to protect the transistor (and chip) from damage when the load is switched off.....

There's a nice illustration next to the text....

Forget it, I drew you a picture and converted the circuit to NPN (No guarantees this will function, as I'm just copying someone else's circuit...) The right side is "their" drawing, the left is the timer circuit from yours...
npn.jpg
John Smith (author)  gmoon9 years ago
Ok, thanks for clearing that up for me. I just get confused with electronics and stuff. Oh, and thanks for the schematic, too.
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