Cigarette-lighter Battery Charger

This project doesn’t use a microcontroller, nor even the slightest specialized integrated circuit (IC). In spite of this, it will look after your batteries, especially if you are reasonable about the charging time. Whether they are old nicads (NiCd) – becoming extinct these days because of their many shortcomings and toxicity – or the omnipresent nickel-metal-hydrides (NiMH), these types of battery have to be charged at constant current.

Resistors
R1 = 820Ohm
R2 = 1kOhm
R3 = 4kOhm7
R4 = 56Ohm
R5 = 10Ohm
R6 = 4Ohm7
R7 = 1Ohm5
Semiconductors
D1 = 1N4004
LED1 = LED, red
T1 = BC557
T2 = BC547
T3 = TIP20
Miscellaneous
S1 = 1-pole 4-way rotary switch (see text)
4 solder pins
2 9-V batteries
PCB, ref. 080226-1 from www.thepcbshop.com