• Cheap programmable time modules

    05/04/2026 at 12:41 • 2 comments

    These modules can be found on AliExpress for a few cents each if you buy 5 upwards.

    It's a 12x12 mm module with the chip under the blob, typical of other cheapies like doorbell generators. The circuit shows that a resistor "programs" the delay, between 2 seconds and 1000 secondshours. It is powered by 2V to 5V so suitable for digital logic. It's effectively a non-retriggerable monostable where a low transition on the input causes the output to go low for the programmed period. There is a reference to a selectable multiplier. I think this refers to the solder jumpers which can be used to increase the delay by 8 to the power of 0 to 3. The documentation is very scarce, there is no formula provided for calculating the resistance required. A reviewer said just as much. I'm putting a link to this on the stack in the hope that someone knows more about these.

    You'll need to buy $15 worth of stuff for free shipping so they could be used to top up an order to reach the threshold. I might get a few next order.

  • A gotcha with SDCC and stm8

    03/28/2026 at 05:50 • 0 comments

    When I updated my SDCC version from 4.2 to 4.5 I was vaguely aware of a change in the calling convention for STM8 MCUs, so I rebuilt the ST Standard Peripherals Library following instructions that I had developed a few years ago so that the library would have the same calling convention as my application code.

    What I missed was that I had an inline assembler routine to delay a few µs. Since the calling convention had changed the first argument was passed in A, not on the stack. The effect of the error was that I was seeing occasional random glitches in the display due to the 74HC595 not getting enough time to load bits some of the time. The following one line deletion fixed that:

    diff --git a/display.c b/display.c
index 34ed872..31bb77c 100644
--- a/display.c
+++ b/display.c
@@ -153,11 +153,11 @@ static void display_fill(void)
 }
 
 // taken from https://github.com/unfrozen/stm8_libs/blob/master/lib_delay.c
+// modified for calling convention 1
 static void delay_usecs(uint8_t usecs)
 {
        usecs;
 __asm
-       ld      a, (3, sp)
        dec     a
        clrw    x
        ld      xl,a

    You can find details of the calling convention, versions 1 and 0, in the SDCC manual, part of its documentation.

  • A dumb mistake with KiCad symbols

    02/01/2026 at 06:00 • 0 comments

    The TL431 programmable shunt regulator is a useful 3 pin chip that I use a lot. In a recent project I used it in the heart of a constant current circuit. I chose the symbol TL431DBZ which uses the SOT-23 package. Most likely I initially designed the board for PCBA and later decided to hand assemble it and use the TO-92 package. I thought it wouldn't matter since I would change the footprint to the TO-92-wide footprint. Of course I was mistaken. Here is the pin table for the TL431DBZ.

    And here is the pin table for the symbol I should have chosen, the TL431LP:

    Notice the pin number assignments differ. The result was that the pads in the TO-92-wide footprint weren't the electrodes I was expecting.

    Fortunately 3-pin TO-92 packages can be rotated to give a different permutation of the pins and this is the hack I will use for the existing boards. The KiCad project has of course been updated.