Simple Relay timer

[craigey1]

Hi all,

I need a circuit that will operate a relay for aprox 10 seconds when power (12V) has been applied. After the 10 seconds the relay should switch back. Power will contine to be applied, but the relay shouldn't switch over again, until the power has been disconected & reconected again.


I know this should should be a simple circuit, but I can't seem to get the relay to switch over for 10 seconds. I was trying to avoid a 555 circuit, so I can make this on a strip board.

This is to fix a problem with my car.
I'm currently using a switch everytime I start the car, which disconnects the coolent sensor, then start as normal, before toggling the switch again to reconnect the sensor.

Any help would be greatly apreciated.

 

[Nigel Goodwin]

Hi all,

I need a circuit that will operate a relay for aprox 10 seconds when power (12V) has been applied. After the 10 seconds the relay should switch back. Power will contine to be applied, but the relay shouldn't switch over again, until the power has been disconected & reconected again.


I know this should should be a simple circuit, but I can't seem to get the relay to switch over for 10 seconds. I was trying to avoid a 555 circuit, so I can make this on a strip board.

I don't see your problem?, 555's are perfect for stripboard, and an obvious simple solution to your problem.

 

[craigey1]

It was more of a case of it being a long time since I've done any circuit design. I know 555's are fairly straight forward, but I can't remember how to work out the time delays etc. I thought it would be easier to use a resistor & capacitor to create the delay.

 

[Gayan Soyza]

Ya 10SEC delay is fair with 555 IC.Basic monostable arrangement will do the job.

To calculate the resistor & cap values.

Info about MiscEl

 

[Hero999]

I think the problem you're having is that you want the relay to turn on when the circuit is powered, then turn off after 10s. Normally a 555 requires a trigger pulse but this can be disabled and the whole IC can be used as a schmitt trigger.

For about 10s R should be 1M and C should be 10:mu:F

The only problem with this circuit is, it needs to be off for awhile to discharge the capacitor but there's no way to avoid this.

**broken link removed**

 

[craigey1]

That's great. I see what you mean about having to wait for the capacitor to discharge before the circuit can be reused, but this shouldn't be a problem for what I need.

Once the car is started the coolant sensor can be connected again & the car runs fine. It only needs to be disconnected when the car is started from a cold (several hours since last being used) start. If I stalled etc, the engine would still be warm enough to not need the coolant sensor to be disconected.

Thanks for all the replies.

 

[MrDEB]

Just hook the 555 to coolant sensor

If car is cold the 555 triggers for 10 seconds.
If warm the 555 is disabled.
or go the other way. Have the coolant pump or fan off , start the car then the coolant fan comes on after the engine is running for 10 seconds regardless if engine is hot or cold. 10 seconds is minor. Might be easier.
The 555 is triggered off the ignition system. I would put a 30 second delay so engine is actually running.
Sounds like your coolant fan is preventing the engine from starting?
If so they do make a temp switch for the coolant fan that only turns it on when needed.

 

[craigey1]

It's not a problem with the coolant or fan. I'll explain..

The car is a diesel & for some reason if it has been sat for several hours or overnight, the car will take aprox 30 - 45 seconds of turning the engine before the car finally starts. I spoke to the dealer & they told me the problem was the fuel pump & wanted £1800 for the part + fitting.
I posted on a forum which specialises in these cars & someone suggesting trying to unplug the coolant sensor, as this forces the car to think the outside temperature is cold, so it turns on the glow plugs. (Most modernish cars don't use the glowplugs unless it's very cold & instead rely on the engine compression to start). After a few seconds the glow plugs turn off & the car can be started, the engine then kicks into life immediately.
The person on the forum suggested wiring the sensor to a switch in the car, but I thought there had to be a better way to do it!

I hope that makes sense & explains what I'm trying to achieve. Hero999's circuit should do the job.

Thanks

 

[Hero999]

The only problem with this circuit is, it needs to be off for awhile to discharge the capacitor but there's no way to avoid this.

That's not true.

A diode across the resistor will help to speed up the discharge, when the power is removed the capacitor will discharge through the 555 and the other resistances in the circuit. This is likely to happen very quickly as the 555 draws quite a bit of current. Choose a diode with a low enough leakage so it doens't interfere with the operation of the circuit - use a small signal silicone diode rather than a power rectifier and don't use a Schottky because the leakage will be far too high.

**broken link removed**

 

[craigey1]

That's great. Thanks

 

[MrDEB]

Have you tried unhooking

the coolant temp sensor and see if the car starts?

 

[craigey1]

That's what the circuit will be doing. (disconnecting & reconnecting the sensor). I have unhooked the sensor, that's when the car starts normally.

 

[jimmythefool]

Have you looked in Maplin Craigey? I think they do a kit that will do this for under £10

 

[craigey1]

Thanks, will have a look at their site later today.

 

[Hamoudy]

thanks alot

 

[craigey1]

Hi guys,

I made up Hero999's circuit on a stripboard, but the relay doesn't appear to be doing anything. Is pin 3 on the 555 timer capable of driving the relay?
I have tried removing the diode between the coil of the relay & replaced it with a resistor & LED. This was so that I would be able to see the led illuminate when the relay was switched over & it should then turn off when the relay switches back. However the LED wouldn't illuminate at all.

I even purchased the astable from Maplin, but it appeared that some parts were missing from the kit! (an LED was in the kit, but didn't connect to anything due to a missing resistor). I have tried using the relay supplied in this kit on my stripboard, but it too doesn't seem to do anything.

I have redesigned the stripboard in case i missed something, but I still cant seem to get this to work.

Any help would be greatly apreciated.

 

[Hero999]

If you build my circuit, make sure the diodes, polarised capacitors and IC are connected the right way round.

Never remove the diode in parallel with the relay, it's there to protect if from the high voltage pulse generated when the relay turns off.

It sounds like you might've got the protection diode backwards, did the 555 get hot?

What current does the relay draw?

The maximum a 555 can supply is 200mA.

Sometimes the -0.6V generated by the relay coil can interfer with the 555, adding a diode in series with the relay and protection diode might help.

 

[craigey1]

I may have got one of the diodes around the wrong way as the 555 did get hot.

However I rebuilt the circuit completely & still the relay wouldn't fire.

I'm starting again! I've tested the relays & one of them was faulty. I'm going to try & adding the 3rd diode & will keep my fingers X'd.

Thanks

 

[Hero999]

As your 555 got hot it's probably been cooked as a huge current flowed though pin 3 and the protection diode.

I recommend replacing both the 555 and protection diode when you rebuild the circuit and it'll probably work.

Also the capacitor from pin 5 needs to be about 10nF, not 10:mu:F.

 

[craigey1]

Is there a good way of testing the 555? I tried using the one from the maplin kit, but the circuit's still not doing anything. It's not getting hot this time.
The capacitor on pin 5 in 10nf (just a typo on the diagram).

This is the 3rd attempt & I'm getting pretty frustrated with what should be a simple circuit.

I don't suppose anyone wants to make this up for me & I'll pay for components, materials / time etc??

Cheers