There are many timer circuits available in the internet using IC555 or IC 4060 also using microcontrollers like arduino, or even using only mosfet. I also already made a simple timer using IC555. These are more of hobby based circuit and arduino type circuits are bulky. I need a timer circuit that is programable and accurate with real time which can be uses as commercial.

Recently I came across a programmable multi function timer 35238-MP as attached. I like it because it is compact and programable. I haven't bought one yet, however from the images and videos in youtubes, I could not see using of any microcontroller/microprocessors like components in the board.

I am wondering how does it works as a programmable timer without using any controller?

I want know if anyone has any idea on how it works (just a brief). Or similar timer circuit of such kinds that is compact, accurate and programmable. What components should I used to make such circuits for commercial products?

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  • 4
    \$\begingroup\$ I'd wager there's an IC (perhaps even a complete PCB) beneath the 7 segment display. \$\endgroup\$
    – Mat
    Jun 4, 2021 at 10:54
  • 1
    \$\begingroup\$ I suspect @Mat is bang on the money, the unpopulated 4 pin connector could well be for flashing a part with a bed of nails in the factory. \$\endgroup\$
    – Colin
    Jun 4, 2021 at 11:00

2 Answers 2


You can see other versions of this circuit where the MCU is visible. In one I found it is an STM8S003F3P6.

In your case, it's hidden under the display (there's plenty of room for that), as @Mat suggests.

From the lack of a crystal, the accuracy is not great (a few percent) since they are using the internal RC oscillator.

Also there do not appear to be any resistors in series with the LED drive pins, which is a bit dubious.

On the other hand, the opto-isolation is a nice feature that may (depending on whether you use it properly or not) allow the circuit to work reliably despite switching a nasty load that produces a lot of EMI.


The first words of the first sentence of 35238-MP datasheet says it is microprocessor based.

So the assumption it is done without using any controller is wrong.


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