Need a super simple 3v DC timer circuit

[supak111]

Hey everyone I'm running a small 3V DC motor 50ma on a regular on/off switch. I would like to replace a switch with a super simple timer circuit to stop the motor after 2-4 minutes so that I don't have to go back and turn it OFF every day. I forget sometimes and it kills my batteries. I would like this timer circuit to be super super simple. Device I'm controlling is cheap and nothing fancy so I don't care if the circuit is designed bad as long as it does the job and I don't have to waste 3 hours making the circuit.

Would adding a NPN transistor with a large cap on the gate do the trick? If I add a momentary switch from battery to cap/gate, cap would fill up and when I let go of the momentary switch the transistor should conduct until the cap discharges right?

How big of a cap would I need to run a 3v 50ma motor for 4 minutes? Is it even possible? I have bc337-25 transistors sitting around.

PS. dc motor doesn't need to run at constant speed the whole 3 minutes, just as long as it runs for around that much time. I was looking to buy something but there is nothing out there that is small and cheap.

 

[davenn]

do some googling on 555 timer circuits

 

[Colin Mitchell]

You will need 3 stages. Use BC338 in place of BD679 and motor goes in collector of BC338. Use 100u Remove diode and 1M and put switch in place of 1M. Remove globe.

 

[Martaine2005]

as it does the job and I don't have to waste 3 hours making the circuit.

Personally, I thought making something was all part of the fun. Not wasting time at all.
I would definitely go the 555 timer route.
There is a wealth of info out there on these very useful chips.

 

[supak111]

No I like making electronics, I just didn't want to invest too much time in this particular project. What the simplest way to make a time with a 555 chip

 

[supak111]

Wait 555 timers are 4.5V-18v, I only have 3v max

 

[Martaine2005]

This example I found in 15 seconds on google. It's here..http://electronicsclub.info/p_timer.htm
The buzzer can be replaced for your motor. Although a transistor might be required from pin 3 to drive the motor.

But my point is, a little research and reading can find the the answers to just about anything.
BTW, the above circuit is an adjustable timer circuit from 1 min to 10 minutes..

 

[Martaine2005]

Wait 555 timers are 4.5V-18v, I only have 3v max

That's the input voltages they can be powered by.

You said ,you are running a 3v 50ma motor. Not that you have fixed 3volts powering a motor.

 

[poor mystic]

Wait 555 timers are 4.5V-18v, I only have 3v max

Hi
You can get CMOS 555's which run as low as 1.5V.
Google "CMOS 555" and you'll see... the part number is LMC555

 

[supak111]

Oh cool. I will try to make something with LMC555 then.

 

[Colin Mitchell]

You will need a buffer transistor as the chip will only deliver a few milliamp.

 

[poor mystic]

Possibly a MOSFET would be a good choice of buffer. MOSFET's have an advantage, at low voltages, over bipolar transistors - there is no 0.6V bandgap to use up your operating voltage ceiling.
Here's a link http://www.electronics-tutorials.ws/transistor/tran_6.html

 

[Colin Mitchell]

A MOSFET needs at least 1.8v

 

[poor mystic]

A MOSFET needs at least 1.8v

OK, but that need not be subtracted from the supply voltage. The full supply voltage is still available to the load, with a MOSFET.
With a bipolar transistor, 0.6V is taken up by the base-emitter junction, which reduces the voltage available o drive the load.

Also, the statement that a MOSFET needs at least 1.8V isn't correct.

 

[Colin Mitchell]

"With a bipolar transistor, 0.6V is taken up by the base-emitter junction, which reduces the voltage available o drive the load." What rubbish.
The collector-emitter voltage can be as low as 0.3v to 0.5v and some ZTX transistors can be as low as 0.05v.

 

[Arouse1973]

I think Poor Mystic was talking about a common collector buffer where approx. 0.6 Volts is lost due to the Vbe junction.
Adam

 

[Arouse1973]

A MOSFET needs at least 1.8v

It depends on the MOSFET. MOSFET turn on voltage is specified at generally 250 uA Ids. Some can have a turn on voltage as low as 0.2 Volts but you will only be able to draw 250 uA at this voltage. It's always best to look at the data sheet for the specific Vgs versus Ids of that MOSTFET.
Thanks
Adam

 

[Colin Mitchell]

"I think Poor Mystic was talking about a common collector buffer where approx. 0.6 Volts is lost due to the Vbe junction."
0.6v doesn't even apply to an emitter-follower because the LMC555 does not go rail-to-rail and you lose another 0.3v.
You don't think of using an emitter-follower when you cannot afford to lose any voltage.