Timing without a 555...

[Externet]

A noise/vibration/motion sensor amplifier should trigger the gate of a mosfet to turn on a lamp, and stay on for about a minute after the sensor detects nothing. Sort of a VOX

If the gate-to-source has a selected capacitor in parallel plus a bleed/pull down resistor; would the mosfet turn off after some time if there is no more triggering from the sensor ?

Is there a dedicated IC that will accept a microphone/sensor and output a (DC) trigger to a mosfet gate ? Or a dedicated VOX IC ?

 

[alec_t]

If the gate-to-source has a selected capacitor in parallel plus a bleed/pull down resistor; would the mosfet turn off after some time if there is no more triggering from the sensor ?

Yes, but the turn-off time would be dependent on the Vgs threshold and hence on the particular FET. If accuracy/stability is not too important then that would work.

 

[Diver300]

If the gate voltage changes very slowly on the MOSFET, the lamp could take a long time turn on and off, which would result in a lot more heating of the MOSFET than if the transition were faster.

You might need to have one MOSFET for timing, and then some Schmitt trigger to turn the power MOSFET on and off quickly.

 

[alec_t]

What is the lamp rating?

 

[Externet]

Half a watt white LED, powered by 18650 lithium mounted on bicycle. Plan is turns on when bicycle moves and noise/vibration triggers mosfet. To stay on at a red traffic light, but turn off ~1 minute after parking. The contraption made/mounted so cannot be stolen.
Another for a rear light, smaller red led.
My daughter commutes on it and sick of stolen lights. Removing when parking is not an option.

Got clues on a VOX chip, a MC2830 ----> https://www.electro-tech-online.com/attachments/mc2830-circuit-2-gif.35091/ and being digested.

 

[MikeMl]

Why not put a micro switch under the seat? Let her weight turn on the circuit.

Here is the story about dissipation in the NFET: (Not enough to even need a heatsink).

 

[Externet]

Like it !
As the bike is 800 miles away, I wanted to send a built thing. Installing a switch at any seat spring gap means traveling... Some day then. Will ask her to send me the seat, as winter is not riding time...

 

[alec_t]

How about using one of these per light? No amp needed. Minimal electronics.