Found in a 1970 EEE, this circuit can be applied to TTL ports. The supply is 5 V and the frequency, which depends on the crystal, can be between 1 and 3 MHz.
This circuit was found in a 1987 edition of the United States' Hands-On Electronics magazine. The...
We found this circuit in an old English publication. The resistor not indicated can be 2k2 and the...
This circuit was found in a Popular Electronics from the 90s. It uses a LASCR which is a photo-SCR...