12VDC Negative Voltage DC Converter by 555

The circuit is very simple, and is easily made on Veroboard or similar. Construction is not critical, and the schematic is shown in Figure 1 below.
Rectifier diodes should be ultrafast (UF4004 or similar), or you can use 1N4148 signal diodes. Losses will be slightly higher if you use signal diodes, or lower if you wanted to go to the trouble of using Schottky diodes - the latter are not warranted in such a simple circuit (IMO). The zener diode is to protect the circuit against transient overvoltage, and is optional
Using only a standard NE555 timer and a few other parts, this circuit should be up an running in about an hour. The 555 is configured as a minimum parts count astable (i.e. no stable states) multivibrator, and runs at around 17kHz with the values shown. The zener diode (D3) should be a 16V/ 1W type. Resistors are 1/2W carbon film, and small caps may be polyester or mylar

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