# Time delay circuit

Discussion in 'Homework Help' started by sky, Jan 3, 2005.

1. ### sky Thread Starter New Member

Oct 13, 2004
4
0
I would like to build a simple circuit with a time delay function (3sec), which a signal is input and then a singal will be output after 3 sec.

requirments:
input 12V
output 12V
can use the resistors, capacitors, transistors and diodes only.

Can you help me? Thanks a lot

2. ### Brandon Senior Member

Dec 14, 2004
306
0
Thats a fairly simple citcuit. You will be using a transistor connected to your input signal to drive the circuit. The transistors emmiter connected to a capaciitor resistor circuit. 1/RC = 5 seconds. Pick to values for R and C. 50-100uF for C would be just fine.

Take the voltage of the capacitor connect to a difference amplifier. If you use CMOS for the difference amplifier you will be fine. If you use BJTs for the different amplifier, use a darlington configuration to minimize the leakage from the Cap. Use 3/5 of 12 volts as your reference level via a voltage divider. This is why you pick 1/RC to be 5 seconds. It would take 5 seconds to get to 12 volts. @ 3/5 of 12 volts it will take 3 seconds to get there. Take the output off of the difference amplifier from the collector of the transistor connected to the reference voltage.

You will need to do tweeking afterwards as no timing circuit will be perfect when constructed. I would use a resistor pot for the R in the RC circuit to modify the time constant as you need to.

3. ### sky Thread Starter New Member

Oct 13, 2004
4
0
Thanks, Brandon.

But could you mind to explain with a simple diagram, Thanks.

4. ### dragan733 Senior Member

Dec 12, 2004
152
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In the attachment I give you an example of that time relay. But this time relay is not very precise because of the great tolerance of the capacitor electrolyte and the pass moment of the signal from 0 to 12V is not abrupt, because the time 3s is great . This circuit is applicable at the times about ms. To get times precise, about seconds, and with pass abrupt of the signal from 0 to 12V it is necessary to use dividers.

5. ### Brandon Senior Member

Dec 14, 2004
306
0
Sorry. I'm not big on giving schematics especially when circuits involved are things we learn in circuits I.

Not trying to be bitchy or snobbish, but when someone gives you a schematic designed already, I feel all you learned is how to come to a forum and ask for a circuit. Yes you may understand how it works afterwards, but each circuit you build will teach you more than anyone could explain by showing you.

6. ### anichur New Member

Jul 7, 2008
1
0
hi
all is it possible to make time delay using 2 transistor resistors a capacitor .
Anichur