Timer Suggestions Needed

Thread Starter

gregeeh

Joined Oct 19, 2021
7
Hi all,

I require a simple cct that will activate a small relay for 5 secs 2 secs after power is applied to the circuit. Maybe a 555?

TIA

Greg
 

MrSalts

Joined Apr 2, 2020
712
The PNP transistor discharges the capacitor when the switch is opened (the timer is turned off. It is important to discharge the cap so you get the same 5 seconds of delay after each switch throw that powers up the 555. The 555 can directly drive many relay coils if you have a 5 or 12 v coil under 200mA.

4D660A06-63F9-4FCF-8186-F5B9E2B4ABF4.jpeg
 

crutschow

Joined Mar 14, 2008
28,170
Below is the LTspice simulation of a circuit using one CD4093 quad 2-input Schmitt-trigger package, and an N-MOSFET driver for the relay:
The relay turns on (yellow trace) about 2s after the power turns on (green trace) and turns off about 5s after that.

1634707170251.png
 

crutschow

Joined Mar 14, 2008
28,170
The PNP transistor discharges the capacitor when the switch is opened (the timer is turned off. It is important to discharge the cap so you get the same 5 seconds of delay after each switch throw that powers up the 555. The 555 can directly drive many relay coils if you have a 5 or 12 v coil under 200mA.

View attachment 250668
I don't think that circuit will work since, if a real 555 TRIG input is held low, I believe the output will stay high.
What generated that displayed output?
 

Thread Starter

gregeeh

Joined Oct 19, 2021
7
@crutschow - Thanks for your cct, it's appreciated. Could the circuit be simplified if I did not require the 2 sec delay after power is applied? ie: Apply Power | turn on relay for 5 secs | turn off relay until power removed & applied again.
 

Sensacell

Joined Jun 19, 2012
2,917
Below is the LTspice simulation of a circuit using one CD4093 quad 2-input Schmitt-trigger package, and an N-MOSFET driver for the relay:
The relay turns on (yellow trace) about 2s after the power turns on (green trace) and turns off about 5s after that.
I love the CD4093 chip! it's my all-time favorite part in fact.
With a 4090 and a handful of resistors capacitors and diodes - you can solve almost any tricky, annoying problem.
 

MrSalts

Joined Apr 2, 2020
712
I don't think that circuit will work since, if a real 555 TRIG input is held low, I believe the output will stay high.
The 555 triggers on power up in mono-mode. It is a challenge to prevent the triggering on startup.
What generated that displayed output?
Closing the switch near Vcc causes the pulses in the image above.

I think you contributed to this thread on the 555 powering up / self-triggering
https://forum.allaboutcircuits.com/threads/555-timer-getting-triggered-on-power-up.158538/

Note: the OPs timing requirement was not perfectly clear.
 
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crutschow

Joined Mar 14, 2008
28,170
Could the circuit be simplified if I did not require the 2 sec delay after power is applied? ie: Apply Power | turn on relay for 5 secs | turn off relay until power removed & applied again.
Yes.
It would eliminate a resistor and capacitor on the CD4093 circuit, or allow the use of a simple 555 one-shot circuit.

If the 5 sec turn-on time is not critical, you could use a circuit with just a MOSFET (below):
When power is applied (yellow trace), the N-MOSFET's gate (red trace) is charged to 12V through the capacitor, turning on the transistor and relay (green trace).
When the capacitor discharges through R1 to below the MOSFET's turn-on gate voltage, the transistor and relay turn off.
The on-time will vary some with the particular MOSFET you use, and the tolerance of C1 and R1, so you may have to tweak the values of C1 and/or R1 to get the desired on-time.
Increasing either value will increase the on-time.

1634736011129.png
 

Thread Starter

gregeeh

Joined Oct 19, 2021
7
If the 5 sec turn-on time is not critical, you could use a circuit with just a MOSFET (below):
I like this, it is really simple and will be reliable and robust. Does not have the ON delay but I believe I can live without that. Thank you once again.

Greg
 

crutschow

Joined Mar 14, 2008
28,170
Does not have the ON delay but I believe I can live without that.
If you can't, an added transistor delay will provide that (below):

Edit: Note that this and the previous circuit take no (only a small leakage) current from the supply after time-out, as both M1 and M2 are then off.

1634829702612.png
 
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