I am building an additional light for a storage room that needs to operate from 12 volts and needs to shut off after 10 minutes.
There are already some lights but they are not bright enough. The new light will be connected to the existing light's power source, but consumes more power and since people only enter to take or deposit something, it should not stay on for longer than 10 minutes (if someone forgets to turn the light off).
I checked some timers available on the market, but none seemed to do what I need, so I decided to build my own circuit.
MY_LAMP is where the lamp will go. My problem is that the capacitor will remain charged for quite a while, thus reducing the shutoff delay the lamp is turned on next time. I need a something to discharge the capacitor in 10 seconds. I tried adding a resistor in parallel with the capacitor, but if I want to keep the 10 minute delay, the discharge time is still minutes. I came up with the idea to have a smaller resistor in parallel with the capacitor, that gets disconnected when power is applied to the circuit.
The things closest to achieving this I could find were depletion-type MOSFETs. But those need a negative voltage applied to the gate to open, which I do not have present. So my question is: what do I need to put in the place of "Some Device" that opens what is now marked 'SWITCH' if 'INPUT' is present. Otherwise the switch should close to discharge the capacitor. And all this preferably without any moving parts, so no relay.

The values of some components, are not yet determined, because I do not know the exact power consumption of the lamp yet.
Or if someone knows an easier design that can achieve what I need, please share it with me.
Thanks for the help.

 

I checked some timers available on the market, but none seemed to do what I need

I'm surprised. Plenty of 'off delay' timers are advertised.
Welcome to AAC!

 

I'm surprised. Plenty of 'off delay' timers are advertised.
Welcome to AAC!

Yes, but all I could find had to be started by a button press and not just a switch turned on.

 

You could use the normally-closed contacts on a relay.

You could also use a diode and transistor such that when the transistor is off (power not applied) the capacitor discharged through the diode. But when power is on the transistor pulls up the resistor to above the capacitor's max voltage and reverse biases the diode.

 

If you discharge C1, your lamp will stays on forever, if the switch (power supply) is left on.

I think the easiest solution is to get an arduino. Its pretty cheap nowaday

 

If you discharge C1, your lamp will stays on forever, if the switch (power supply) is left on.

But C1 will only be discharged while the power is off. If the power is off, the light will be off.

 

You could also use a diode and transistor such that when the transistor is off (power not applied) the capacitor discharged through the diode. But when power is on the transistor pulls up the resistor to above the capacitor's max voltage and reverse biases the diode.

Yes, this would work.

 

But C1 will only be discharged while the power is off. If the power is off, the light will be off.

Power cant be off since 'someone forgot to flip the switch off'

 

Power cant be off since 'someone forgot to flip the switch off'

If the power is left on then the capacitor will be left charged.

 

Seems like the perfect job for a basic 5 pin relay.

 

If the power is left on then the capacitor will be left charged.

Thats why it dont work. The TS wants the lamp to turn off after 10mins, then discharge the capacitor so that the next time the power is switched on the circuit will restart, but if the power is left on 'someone forgot to turn off', C1 stay charged and now the lamp wont turn back on if you turn the off/on switch again.
Now the TS thinks that, if he could somehow discharge the capacitor by 'some device'
after the lamp went off, the circuit will restart as usual, but if the capacitor is being discharged while the power is still on 'someone forgot to turn off', then the lamp will turn back on and stays on.

 

Thats why it dont work. The TS wants the lamp to turn off after 10mins, then discharge the capacitor so that the next time the power is switched on the circuit will restart, but if the power is left on 'someone forgot to turn off', C1 stay charged and now the lamp wont turn back on if you turn the off/on the switch again

It will if you turn it off then on again.

 

Why not just install a motion sensor switch instead of the normal light switch? Job done...

https://www.google.com/search?q=mot...9ajjAhWKXc0KHQ0kD3oQ_AUIECgB&biw=1600&bih=757

 

Yes, but all I could find had to be started by a button press and not just a switch turned on.

I have one in my garage that is motion detect and off timer...something like that might work for you.

 

Here's the LTspice simulation of a 555 one-shot circuit that only triggers when the switch turns on, and either stays on for 10 minutes, or shuts off with the switch, whichever is shorter.
C2 holds the TRIG low for a few ms when the power is applied to trigger the 555.
The 555 output then goes low after about 10 minutes and C1 is rapidly discharged by the DIS input.
Alternately, if the switch is turned off before the 10 minutes (at 800s in the simulation), then D1 and R1 discharges C1 (Vcap).

 

Mr. crutschow got it, never thought of this... silly me

 

Thank you crutschow, that is exactly what I needed. I have one question, though: Is R1 even necessary? I simulated the circuit without R1, and C1 discharged fast enough through the DIS input. Is it only because I was using a less advanced simulator, or is it fine if I build the circuit without R1? I simulated it using http://www.falstad.com/circuit/

 

Thank you crutschow.................
is it fine if I build the circuit without R1?

Call me zapper.
I would not trust the falstad simulator to determine that.
My LTspice simulation, using a transistor model for the 555, shows a slow discharge for C1 without R1 (about 20s to reach 1V).
But you can determine whether you really need it when you build the actual circuit, if using one less resistor is important to you.

 

I would not trust the falstad simulator to determine that.

That's what I was actually interested in
That "get rid of everything that we don't need" instinct comes from me having built a few battery powered projects recently, which were designed to last years. But here it is not that important.

 

R1 exists to prevent a short between Vcc and ground.