Found in a 1980s documentation, this circuit makes use of components from that time. Modern equivalents can be tried out.
The 1 kHz tuned filter shown uses a 741 op-amp. The components can be re-scaled to suit the...
This voltmeter for continuous voltages has several scales and is totally passive. The diodes are...
The sensitive relay reed trigger transistors can be the BC548 and the diode can be the 1N4148. The...