Morse Bug Debouncing

Debouncing a Vibroplex Blue Racer

by DL6ZB, Rolf Heine

If you own one of those beautiful and famous old-fashioned bug keys, you may have noticed a problem when sending Morse dits: contact bounce. Bouncing is a fundamental problem with every switch in electronic devices. Engineers have faced it ever since switches and relays were invented, and several debouncing methods have been developed over the years. One solution is to connect a low-pass filter across the switch (Morse bug) contacts. Other solutions are based on digital methods. I tried to solve the problem in the simplest way possible.

My solution is this: just connect a 330 nF non-polarized capacitor across the bug's terminals. Be careful with the voltage rating of the capacitor if you use the bug with an ancient tube transmitter! Check the manual before installing the capacitor. A keying input impedance of 10 kΩ is assumed, which should be OK for most transceivers. If your transmitter's keying input impedance is far from 10 kΩ, you should recalculate the capacitor value for successful debouncing of your mechanical key.

A good idea might be to use a 220 nF capacitor across the bug terminals and another 100 nF capacitor inside a standard 6.3 mm (1/4-inch) key plug. This provides some additional RF filtering at the transmitter's key jack.

Here are the results of this debouncing method, which works fine at speeds of about 20–30 WPM. At higher speeds, the debouncing method should be improved.

**Picture 1:** Dits from the Vibroplex Blue Racer at approx. 20 WPM without debouncing. The falling edge shows dit bounces.

**Picture 2:** After zooming in, the dit bounces are clearly visible.

**Picture 3:** The shape of the dits after installing a 330 nF capacitor across the bug's contacts. Again, the speed is about 20 WPM. The oscilloscope settings are the same as in Picture 1.

**Picture 4:** Compared with Picture 2, the bouncing is clearly reduced. These small bounces are tolerable for me. The oscilloscope settings are the same as in Picture 2.