Hi I have a 555 delay-off circuit which turns on the output, then after a 3 second delay turns off. Similar to : http://www.learningaboutelectronics.com/Articles/555-timer-delay-before-turn-off-circuit.php

How can I make the output turn on only after a few seconds has elapsed - instead of immediately?

Step-by-step this is what i'm after:
1. Switch power on
2. delay 5-8 seconds
3. switch 555 output HI for 3 seconds
4. switch 555 output LO

 

Hi

You can use two 555s or one 556

 

You didn't bother to mention your Power-Supply Voltage or the Load that You want to switch.
A Comparitor is the way I would do it.
Values are given for 5V and 10V Supply ........
.
.
.

 

I like LQC's circuit.
Below is the LTspice simulation of it slightly modified, using LM339/393's.
I changed both comparator's reference voltage to be the same, derived from a two resistor divider instead of the diodes, so that the delay and pulse times are essentially independent of the supply voltage (trigger points are always at the same relative point on the RC discharge curve).
This can be seen by the times being essentially identical for supply voltages of 5V (green traces) and 12V (yellow traces).

I also set the Ref trigger point at a low voltage, so the limited CM input voltage of the LM339/393 would not be a problem.

 

Except the TS asked for 555.

 

Except the TS asked for 555.

Then he can use your circuit, if that's his preference.

We just suggested an alternate that appears to be somewhat simpler.

 

I have a 555 delay-off circuit

Are you sure? I did not see any circuit attached to your post. It is hard to recommend changes to a circuit that is secret.

Is the 555 you are using CMOS or bipolar?

There probably is a way to do what you want with a single 555. But . . . . . you first.

ak

 

While we're waiting for the TS schematic, here is a variation of something from the Projects forum in 2016. It is the same basic idea as in #3 and #4. There are fewer components because a CMOS gate basically is a comparator with a voltage reference and hysteresis built-in. The CD4093 also will work in this circuit.

R3 assures that there is a discharge path for C1 and C2 when power is disconnected. If you need a shorter reset time, add a small signal diode across R1 and R2; cathode to Vcc, anode to the capacitor.

ak

 

Another version of using two 555's

 

Another version of using two 555's

That a nice simple circuit if using two 555's.
LTspice simulation below:
I had to tweak the RC values to get the desired times.

 

Same basic idea as #8, with the 555 acting as a hysteretic gate.

ak

 

Hi All, I'm open to using other ICs. I just 555s available to use now. Also I am using 12VDC input.
Basically I'm after a circuit using the least amount of components that will do as per post #1.

 

Basically I'm after a circuit using the least amount of components that will do as per post #1.

Go with post #8.

 

Another version of using two 555's
View attachment 261151

Hi sghioto , I like this - will this work for a 12VDC input ?

 

Yes it will.

 

You don't say how much output current you need or what the timing circuit is driving. If the output current requirement is low (like 5 mA), then I recommend changing to CMOS versions of the 555, such as the National/TI LMC555. This part has a much higher input impedance at all pins. With that, you can adjust the two timing networks to use larger resistors and smaller capacitors. For these relatively long time periods, the majority of the output accuracy, and stability with aging and temperature, comes down to the capacitors. For electrolytics in timers, smaller is better.

ak

 

Output connects to a relay driver.

 

Relay coil resistance / power?

ak

 

Same basic idea as #8, with the 555 acting as a hysteretic gate.

Yes.
The main advantages of using the 555's are higher output drive, and more accurate trigger voltages.