Dekatron Pendulums – without using a μC (Click Photo to view Video)

Here are schematics of pendulum circuits that do not require a microcontroller. No programming required. Just wire it up, and play.

120V, 60Hz version (click on image to enlarge):



220V, 50Hz version (click on image to enlarge):



These also make great 1Hz (1 second) line sync’d  timebases.

Here is the timing diagram, for the 60Hz version (click for enlarged view):

Warning: These circuits, as shown, are connected directly to the AC line. They are not isolated, and may pose a shock hazard. They can be electrically isolated, by either using a dedicated isolation transformer, or use a dual (110V) primary transformer, in the case of the 120V circuit. Tie one primary to the 120V AC line, and the other primary (acting as a 1:1 secondary) to the circuit. Leave the actual secondary unconnected (and un-shorted).

Discrete Component Pendulum

(Click on image to enlarge)


This pendulum circuit works using only discrete components. In addition to not using ICs, This circuit “rests” at the “main” cathodes only. It only, transiently, crosses past the “guide” cathodes. The IC based circuits (including the uC based ones) step uniformly at each of the 30 cathodes (guide and main). This discrete one makes only 10 steps per revolution.

CMOS Pendulum, Revisited:

The 4000 CMOS pendulum circuit was designed way back in 2003. It uses two chips. A 4017 counter and a 4013 flip-flop. The circuits above, show it powered directly from the AC line. I also incorporated it into my original descrete logic nixie clock kit, where power is boosted from low voltage.

A new circuit was just designed and tested, in July 2016, that only needs a 4518 dual counter chip. Here is the circuit (click on drawing to enlarge):


Even though, the output of the counter, is encoded, the timing diagram shows how the guide signals are extracted. /G2 is pulled directly from QB. It only has a simple RC circuit, to filter out the transient “count reset” pulse. G1 is a bit of “magic”. Its held “off”, by connecting the emitter of Q4 to QB. That way /G1 can only be activated when QB is low. To determine if it comes before of after /G2, an exclusive-OR gate is made around Q5, with steering from D3 thru D6.  The other half of the counter is only used for its QA stage, which acts as a toggle flip-flop, determining the direction, of the glow (F/R). Its toggled by the index (NDX) signal, when the glow reaches cathode K0. This actually generates several closely spaced pulses. On the old circuit, a one-shot was added to output only one. Here a very low R4, and C2 make short pulses invisible to Q2, so only one pulse comes to /SWAP.

This curcuit has not been fully analyzed to see, if it makes a reliable time base, such as the older circuit.