switch a relay after a short delay

Hi,
I know very little about circuit design so I wonder if someone would mind helping me?

I would like to produce a circuit that lets me connect a wire to earth and disconnect it again.

If I don't reconnect it again within a few seconds a relay will close and stay closed until reset from another lead connected to earth.

If I connect it momentarily again it resets the timer. I.E. as long as I keep connecting the wire (pushing a button) the delay keeps reseting, when I stop for a few seconds, say four seconds the relay closes.

Picture people walking through a gate, after the last one has passed through the sensor triggers.

I can reset the relay by closing another push button, i.e. a relay latched with one of its own contacts set at the point between the coil and above contact and reset with a wire between the v+ and a resistor at the other end of the coil.

Could an rc timer do this with a transistor?

If the circuit turns the relay on immediately and off after the delay this might also work, I could trigger two relays but that would make a larger circuit.

What are the specs on your relay? Coil voltage? Coil Current?

Hi,
I can use any one of a number of relays.

I have 12v 150Ω or 24v 620Ω relays to hand.

I was willing to experiment with the voltages and resistances.

I have tried to enclosed a rough drawing of what I have in mind, I don't know if it is readable until I submit this.

If it is , I want to either connect something to point A that will delay the coil resetting and be able to earth B so it sets (preferred) or earth A to reset and put something onto point B with a delay to set the relay.

Altering the resistor above point A allows me to use different value relays in the circuit.

Thanks.

If I've understood your requirements this should do the trick:-

This is for 12V. 24V would be too much for the 555 IC.
Time constant R3C2 determines the delay before the relay energises. R3 could be replaced by a potentiometer. R2 and R4 limit the C2 discharge current.

Hi Alec,
I have built and tested the circuit you kindly sent me.
It is not quite what I was looking for.
In your circuit if I press the reset the timer starts, if I keep pressing the trigger button the timer keeps restarting.
If I press both buttons and keep pressing them the time keeps restarting.
when the trigger button is not pressed for a few seconds the relay sets.

I want the relay to switch from a green light when the timer is reset (the reset button) to a red light and stay on red until the trigger button is pressed, this then starts the timer and nothing moves until it times out, then the relay sets and the green led lights and the red goes out.

I can put your circuit in the present line. When the present circuit goes back from red to green the contacts will be via your relay and will quickly set it back to red as both buttons are pressed (I did notice that if I pulse both buttons the result is the same as just pulsing the trigger)your circuit will then keep it on red until it times out and goes back to green.

I can use this unless you can think of a way of reseting with one button and making the other button start the timer but do nothing until it times out.
I hope you can understand this, thanks for your help,
Bob.

Understood. I'll work on it.
Are your red and green lights just normal LEDs? If so, you may be able switch them more efficiently with the 555 or other IC or transistors instead of using a relay (which draws more current).

Edit:
Here you go. I think this is what you want. The sequence is more complex than I originally understood you to want; hence the use of a second 555. You could, of course, use a 556 instead of two 555s, and the LEDs shown are by way of example for lights. Vary R2 and/or C1 if you want to adjust the timing.

switch a relay

Hi Alec,
Sorry about the delay, I have been meaning to purchase a 556 for a while, I got chance yesterday and built the second circuit you sent me today.
It is perfect and exactly what I wanted, I owe you a debt of gratitude.
I really am grateful for the help you have given me.
The circuit will eventually run a larger circuit but the small relay on the circuit board will be the interface to the final circuit and it will run from an external 24 volts so current is not too much of an issue.
I have sent a photo of both of the circuits set up on blob boards.
Again thank you.

Bob.

Nice to know it works as you wanted.