The transistors of this circuit found in a 1965 magazine can be the BC558 and the sensitive 12 V x 50 mA relay. The other components are common.
To obtain a signal with a variable mark / space ratio in a ratio of 1 to 20 to 20 to 1, we have...
Other operational amplifiers series, such as the MC34072, MC34074, MC35071, MC35072, MC35074 d...
We found this sophisticated meter circuit in an article in Radio Electronics magazine from June...