This simple circuit will produce an audible tone whenever the ZX80 screen is blanked, yielding a click to indicate key closure, or a steady tone during processing.
Examination of the “SYNC” line (IC 19—PIN 5) with an oscilloscope reveals three constituent signals: Line Sync (denoted LS); Frame Sync (FS); and Keyboard (KBD). These signals are low assertion, as indicated, and have period and repetition rates as follows:
Signal Period Rep. Rate LS 6us 58us FS 380us 18ms KBD 6ms N/A
(LS, FS and KBD are printed with overbars to show low assertion.)
The time constant of the RC low pass filter was chosen to allow the second gate to switch only on signals long with respect to FS, i.e., KBD. When SYNC returns high, the first gate sinks the discharge current of the capacitor via the Germanium diode, resetting the circuit. Germanium is used here to insure that the minimum negative-going threshold voltage (VT < 0.6V) of the second gate is reached, turning off the buzzer. The Schmitt-trigger was chosen for its high positive-going threshold, enabling a less critical circuit design than would be possible using standard TTL.
The buzzer (a piezoelectric job from Radio Shack #273-064) drive circuit is taken directly from the blister pack except for the diode added to PIN 8 to clamp positive oscillations to the five volt rail.
A 74LS121 one-shot could be used instead of the 74LS14 to eliminate the steady tone during processing, but I prefer having the sound as an indication of processing activity.
The Schmitt-trigger was “piggy-backed” on IC 19, picking up power and the common (PIN 5) SYNC signal with a dab of solder. I used that “double sided stick ’em stuff” intended for wall hangings to mount the buzzer across two other IC’s. The balance of the circuit was “sky-wired” and the entire modification was neatly fitted within the standard ZX80 case, so as not to obtrude in an obvious manner. A bit of insulating tape may be needed on the inside of the cover to prevent shorts.
The audio feedback has made life with the ZX80 passive keyboard enjoyable and has freed my attention to concentrate on what—as opposed to how—I am typing.
[Figure: Circuit Diagram for the Key click Generator — SYNC into a 1/6-74LS14 Schmitt trigger (pins 5/6), 1K5 resistor, germanium diode and 1µF capacitor into a second 1/6-74LS14 (pins 9/8), driving a general-purpose NPN transistor (470Ω, 10K, 220K) and the piezo buzzer from +5VDC]