External Trigger for Stop Watch




About: I'm an Engineer. I like hiking, flea markets, and electronics.

This instructable will show how to add an external trigger to a stop watch so that it can be used for event timing from circuits. It's designed to accept from 1.5 - 50 VDC, though I've only tested it from 1.5 - 9 VDC.

Step 1: Dissemble Watch

Take the watch apart.

Step 2: Route Switch Wires

Add wires to the start/stop switch. Check the polarity of these wires and mark them with a sharpie to keep track.

Step 3:

Solder the following circuit. Here's how it works:

When there are 0V across RED-BLK, the transistor is non-conducting.
When there are > 0.6V across RED-BLK, the transistor conducts.
When there are < 0.6V across RED-BLK, the diode protects the transistor from reverse breakdown.
The 10K resistor limits current into the base.

Transistor turn-off speed isn't critical in this case, but if it was, I'd recommend adding a 1K resistor in front of the 10K resistor and a 10-100nF capacitor in parallel with the 10K resistor. This would help to quickly add to, and remove base charge to the transistor.


The flash card with the transistor on it shows how to figure out the pinout of an unknown NPN transistor. Using a multimeter in diode test mode, you'll find that the base-emitter junction has a higher potential than the base-collector junction.

Step 4: Add Prongs & Finish

In this step, add some screw prongs by drilling holes slightly smaller than the machine screws you will be using. After, reassemble the watch.



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    3 Discussions


    4 years ago on Introduction

    Thanks for the post. Appears that the external trigger of 12 volts (constant, lets say a motor running), would start the stopwatch upon application of the trigger voltage. This constant voltage would maintain transistor in conductive state. When the 12 volt trigger voltage went away (motor stops) the clock would continue to run, stopping only when the 12 volts was re-applied to the trigger. Is this accurate? I am not an electronics guru. I ask because I have made my own stopwatch modification which does indeed 'pulse' the start/stop contacts of the stopwatch upon external 12v power on/off. It uses relays and capacitors but if there is a way to do it with transistors, it would be much preferred. Yours seems so elegant and I am hoping my understanding is wrong. Below is my 'final product'


    A microcontroller that I'm wanting to use has a discrepancy between the datasheet timing and the simulator timing so I want to see how much time analog-to-digital conversion takes. I figure I'd be able to put the analog-to-digital conversion inside a loop and then calculate it from there.