Homebuilt Hellschreibers

Posted on Feb 5, 2014

The circuitry for the tone detector, solenoid driver, and motor speed control (paper transport and spindle) is rather straightforward. A solid-state implementation of the driver circuit typically needs a diode (installed with reversed polarity) across the magnet solenoid, to avoid damage due to the inductive voltage spike upon de-energizing the solenoid.

Homebuilt Hellschreibers
Click here to download the full size of the above Circuit.

This is referred to as a flyback, snubber, freewheeling, catch, or suppressor diode. If the relay is a bit slow when energized, you may try to speed it up with one of the simple circuits described here. They basically briefly apply almost twice the supply voltage to the relay coil when energizing. The capacitor in the circuit will have to be adapted to the desired speed and relay (decay time). Paper transport speed calibration is easy: all you need is a stopwatch and a ruler or tape measure. Simply adjust the motor rpm to obtain about 47 cm per minute. Unlike the spindle in a Feld-Hell machine, my spindle is a single-thread (1-start) worm. It needs to turn at 7 columns per character x 2 ½ characters per sec x 60 sec/min = 1050 rpm. The Feld-Hell machine has a double-thread (2-start) spindle: it has two intertwined windings. Note that the number of threads is not related to the number of turns of each thread! Both the Feld-Hell and my homebuilt spindle have two turns. The Feld-Hell spindle only needs to turn at ½ x 1050 = 525 rpm, to get the same effect as a single-thread spindle at 1050 rpm. Adjusting the spindle speed to 1050 rpm can be done several ways: 1) first get the electro-magnet to work properly (which you have to do anyway, hi), then feed the printer with "known-good" Hell-audio and adjust the rpm until the print becomes legible and the text lines are horizontal; or 2) use a tachometer. I used method nr. 2 by...

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