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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

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - 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
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.