12v Delay circuit - Help Please

[pads]

I am trying to devise a delay circuit which will keep the forward facing parking sensors on my vehicle switched on for about 30 seconds or so after I touch the brake pedal.
I have designed the following circuit -


The idea is that when the brake is applied, a 12v input triggers the transistor to switch on and operate the relay which then latches the 12v supply onto the circuit.
As the capacitor discharges, the base emitter voltage reduces, and eventually switches off the transistor, which releases the relay.
Diodes 1 & 2 feed the circuit from the brake 12v, and D3 protects the relay. Variable resistor is to adjust the time of the delay.
Transistor is a BDX33C - darlington NPN which I had handy.

However I cannot get the relay to latch on initially.

Any ideas of where I am going wrong would be appreciated.
Thanks
Peter

[Agent24]

Not sure the problem really but D3 isn't to protect the relay - it's for protecting the transistor from the reverse high-voltage spike generated in the relay coil when it's de-energized.

Not sure the purpose of D1 and D2 though...

Also you might need a current limiting resistor on the relay coil, otherwise you may burn out the transistor.
In fact, that the circuit doesn't work, perhaps this has already happened? Of course I may be completely wrong about that...

[pads]

Agent24 - Thanks for the comments.
You are right about D3 - dont know why I put it as I did!

D1 & D2 are there to allow the capacitor to charge from the 12v trigger (brake pedal), but not from the 12v 'latched' supply.

The relay has a resistance of 680 ohms = 17mA. Since the Ic current for BDX33C is max 10A, I would not have expected it to be burnt out, but maybe.

Thanks
Peter

[Simon (Webmaster)]

Hi,

I'm not sure you need the latching part of the circuit - the input from the brake should charge the capacitor and trigger the transistor & relay, supplying power to the output. when the capacitor discharges the transistor and relay will switch off. I'd remove D2 (so the brake trigger is only connected to D1), and connect the 12V supply directly to the relay coil & D3, omitting the top relay contacts in your diagram - sorry I haven't got time to modify your drawing.

You really need to check the voltages at the C1 / R1 junction and the base of the transistor when the brake trigger is applied. Also try temporarily connecting the transistor base to 12V via a suitable resistor to check if the relay operates. If in any doubt replace the transistor in case it has been damaged.