I built a circuit that pulses a 50-ohm electromagnet when a 'trigger' magnet passes over a hall-effect sensor. This circuit swings a pendulum and simultaneously plays an audio clip as long as the 555 timer output is high.


Pendulum/Audio Schematic

(Link to this design on Circuit Lab website)

This worked perfectly on a breadboard, but when I mounted all components on a small soldered circuit board, the audio module now resets (starts from the beginning, indicating a loss of power) each time the coil is triggered.

I used (4) AA batteries so I could tap the 3rd cell to provide ~5v to the audio module and avoid using a 7805 or similar.

I have an electronic engineering background, but not enough experience to know what may be causing the problem or what modifications are needed. I also unfortunately do not have an oscilloscope.

I am very interested to understand what may be causing the problem and very much appreciate any help the forum can provide.

Many thanks in advance!


Use Twisted pairs for inductive current loop including diode Vcc and FET to Gnd then route these at right angles to sensitive inputs.

enter image description here Keep the area of yellow paths tight and twisted pairs with shortest leads and away from green.

Solid ground connections and Vcc connections with decoupling caps are needed too.

  • \$\begingroup\$ I'm going to source a ceramic decoupling cap today, it would be great if the solution was that straight forward; I'm investigating common values now. I will also ensure that the coil leads are twisted and isolated from all other wires and traces. If that doesn't work I will look into further decoupling the circuits with an opto-isolator. I agree that the issue of proximity is a significant factor, all components are currently condensed into a 2"x3" board so there are many areas to couple noisy signals. Many thanks @Tony EE rocketscientist \$\endgroup\$ – Mike9104 Jan 5 at 13:12
  • \$\begingroup\$ Would a more robust decoupling topology be appropriate for this circuit, including a bulk electrolytic cap and ceramic decoupling cap in parallel with the supply, and possibly even an inline inductor in front of the audio module? If so, are there common values you might suggest for this circuit? It's been a long time since I've done the calculations so I'm doing some research now. Thank you. \$\endgroup\$ – Mike9104 Jan 5 at 13:51
  • \$\begingroup\$ There are so many voltage and current details missing. Without more details. I cant' help. The audio runs @ 1.2V? using a PNP instead of a NPN emitter follower \$\endgroup\$ – Sunnyskyguy EE75 Jan 5 at 16:28
  • \$\begingroup\$ It's not clear how your circuit is triggered Hall or 555? when it works and fails. The audio emitter 1.2V switch is not the best. There are no specs. \$\endgroup\$ – Sunnyskyguy EE75 Jan 5 at 16:43
  • \$\begingroup\$ I wanted to post a followup that I did some experimenting and found a solution that works (though I will also be incorporating the decoupling components you suggested). I put a 10uF electrolytic between the 555 Output and ground that is keeping the base of the 2N2222 high during the voltage dips and is allowing the audio module to play reliably. The audio module is powered by 4.5-5V, which is coming from the 3 AA cells. The 555 activates the entire circuit for a certain duration, then the coil is energized only when a magnet passes over the Hall sensor. \$\endgroup\$ – Mike9104 Jan 5 at 21:24

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