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Project Notes

#507 Single-stage Delay Switch

Comparing BJT and MOSFETs for creating a simple turn-off delay with discrete components.


Here’s a quick demo..



Adding an RC timing component to a single-stage low-side control circuit is a simple way of creating a “turn off delay”.

The delay is proportional to the RC time constant, but the actual behaviour is quite different depending on whether an NPN BJT or n-channel MOSFET is used as the active control component.

In both cases, a capacitor is charged during the “on” period. When power is disconnected (switch off):

  • an NPN BJT will gradually turn off as the capacitor is discharged via the resistor and B-E junction. The current will reduce with voltage, producing a prolonged “fade out” effect.
  • an n-channel MOSFET will turn off as the gate voltage drops, but the ohmic region will be relatively small, producing a sharper off transition

I was inspired to whip up a quick demonstration after seeing learnelectronics’s “How to make a simple delay circuit” video:


Demo Circuit

To compare the BJT and MOSFET behaviour, I put both on a breadboard, triggered by a DPDT switch. I used an 2N3904 NPN transistor, and a 2N7000 n-FET, though the parts selection is not critical.

For visually testing the circuit, the delay is set rather slow with R=1MΩ and C=1µF, for a time constant of 1s.



Actually on a breadboard:



To capture behaviour with an oscilloscope, I switched to R=10kΩ and C=1µF for a time constant of 10ms.

In the following trace, the channels are all offset by -2V and set for 1V/division. Channels are assigned:

  • CH1 (Yellow) - MOSFET switch: MOSFET drain
  • CH2 (Blue) - MOSFET switch: capacitor anode/MOSFET gate
  • CH3 (Red) - BJT switch: BJT collector
  • CH4 (Green) - BJT switch: capacitor anode


Points to note:

  • n-channel MOSFET switch: when capacitor voltage drops to ~2V, the FET rapidly switches off within about 6ms
  • NPN BJT switch: when capacitor voltage drops to 0.9V, the BJT fades over more than 100ms

The Obligatory OCD Build

I felt like flowing some solder, so made the circuit permanent in a very literal transposition of the circuit to a freeform build mounted on a bit of cardboard:


Credits and References

About LEAP#507 SwitchesRC
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LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

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