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Need help designing a timer circuit

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Need help designing a timer circuit

Old 10-16-06, 06:03 PM
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Need help designing a timer circuit

I searched google but couldnt find what i was looking for. I need a circuit that will output 40 ma. at 4.0 VDC every 60 seconds for 2 seconds. So basically once a minute it will send out a 2 second 'pulse.' It can be run off of anything, like a 9volt battery/adapter. Can anyone help me or lead me in the right direction? I can solder it if i get the schematic and parts list.

thanks in advance
Old 10-16-06, 06:13 PM
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What are you trying to blow up?
Old 10-16-06, 06:21 PM
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I'm trying to blow up one planet every minute. luckily i dont have to deal with Pluto anymore...

Its actually for an old pen and ink seismograph. The old clock broke and i need a way to make timing 'tic' marks.
Old 10-16-06, 07:17 PM
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How precise do you need the timing to be? Can it be an analog oscillator or does it need to be crystal based?
Old 10-17-06, 01:13 PM
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Not too precise. my back up plan is to use a second hand of a wall clock to to connect two pieces of wire once a minute, if thats any hint as the lack of preciseness.

I was looking around and it seemed like there are some simple timers that use capacitors and CM555 chips.
Old 10-17-06, 01:18 PM
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Easy. Use a 555 (it doesn't have to be a CM555). You can do this with a 555, 2 (or 3?) resistors and a capacitor. You may need to use an additional transistor to drive the load.

You can buy all the parts at Radio Shack and have it working tonight. Breadboard it first because you'll be playing with resistor values quite a bit until you get it right. Use a fairly large cap (470uF?) for the long cycle time.
Old 10-17-06, 01:53 PM
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Know your subject

Easier to digest info.

You will need to either run your external circuit off 5V instead of 4V or use a resistor ladder on the output to reduce the transistor output voltage level.


Determine what you need:

Calculate Discrete Values for the circuit

The Transistor drive (look at the PNP circuit under "Choosing a suitable PNP transistor"


Here's your shopping list:

Breadboard

555 chip

A Bunch of Resistors

Pick up a few caps

PNP Transistors


A 9V battery, a snap connector for it, and a few alligator clips are nice, too. Throw an LED and resistor on the output for debugging.

The 555 output has a duty cycle > 50%. (effectively a negative pulse). The PNP transistor output will invert the pulse to get a positive pulse for your output.

Play with the circuit on the breadboard and don't be afraid to experiment.

Last edited by Pistol Pete; 10-17-06 at 02:05 PM.
Old 10-17-06, 02:03 PM
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Originally Posted by Pistol Pete
Play with the circuit on the breadboard and don't be afraid to experiment.
Bi-curious Electronics Engineering 101

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