Tiny85 Controlled EM800 Indicator Tube

Mikrocontroller contolling indicator Tube
Not very long ago, tubes were the most important parts in electronic circuits. Nowadays they have almost disappeared, but there are still some fans out there designing really impressive tube circuits, especially tube amplifiers or radio transmitters and receivers.

But one kind of circuits is probably the most popular one: Circuits using tubes that were designed for optical indicaton purposes. There are many beatiful Projects like Nixie Clocks or Scope clocks out on the internet.

One special kind of indicator Tubes are the EM800 'Magic Bar Graph' tubes, usually found in old radios as a tuning indicator.

This circuit here shows how to control such an inicator tube with a modern microcontroller.

The indicator tube is not very hard to drive, just add a heater voltage (6.3V), and an anode Voltage of about 250V.

Depending on the control voltage (0 to -10V) applied to the gate, the bar on the tube will widen or become narrower.

So all you have to do is to provide a negative variable Voltage to the gate in order to controll the tube.

Enter The Controller:

The Schematic

Built around an Atmel Tiny85 is a small inverting switchmode voltage converter that will provide exactly the type of voltage that an EM800 indicator tube needs. Timer0 of the controller is used to generate a PWM Signal of some 20KHz. The Duty Cycle of this PWM Signal directly affects the level of the negative tube control Voltage (0..-10V), thats basically all. Just by setting one value in the OCR Register the length of the tube bar can be controlled by the Atmel Tiny85.

To make this circuit really useful, the Atmel Controller is also programmed to receive bytes on a serial USART interface, so any Computer can easily be used to interface with this circuit.

Just imagine a Raspberry PI displaying its current processor load on this gadget, or take 8 of them and build a really cool Steampunk Spectrum Analyzer or whatever!


PCB and parts at www.askjanfirst.com

Small Videoclip.

Additional Information,Basics,DIY Kit, Links for Schematic
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