Motorola suggests this circuit in an 80's documentation. The circuit uses an inverter to produce a higher voltage for 9 V battery charge from a single cell.
This TTL decoder circuit for PC ports was found in a document from 1992. The power supply can be...
This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even...
The basis of this circuit is a CA3130 operational amplifier. The transistor can be BC548, and the...