The main part of the circuit mobile cellphone charger is timer IC NE555, used to charge and monitor the voltage level. IC1 get control voltage to pin 5 by zener diode ZD1. Threshold pin 6 and trigger pin 2 is supplied with a voltage set by VR1 and VR2 respectively. The trigger pin 2 of IC1 is below 1/3VCC when discharge battery is connected to the circuit as a result flip-flop of IC1 is switched on to take output pin 3 high. The process is reversed when battery is fully charged of charged battery is connected. Here transistor T1 used to enhance the charging current from output pin 3 of IC1. Adjust potentiometer VR1 and VR2 as per require.
LED status for different charging conditions| Load across the output | Output frequency (at pin 3) | LED1 |
| No battery connected | 765 kHz | On |
| Charging battery | 4.5 Hz | Blink |
| Fully charged battery | 0 | Off |
Parts
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 390 Ω
R2 = 680 Ω
R3 = 39 Ω/1W
R4 = 27 KΩ
R5 = 47 KΩ
R6 = 3.3 KΩ
R7 = 100 Ω/1W
VR1, VR2 = 20 KΩ
Capacitors
C1 = 0.001 µF (ceramic disc)
C2 = 0.01 µF (ceramic disc)
C3 = 4.7 µF/25V (Electrolytic )
Semiconductors C1 = 0.001 µF (ceramic disc)
C2 = 0.01 µF (ceramic disc)
C3 = 4.7 µF/25V (Electrolytic )
IC1 = NE555 timer IC
T1 = SL100 or any Medium power general purpose NPN transistor like: 2N4922 , 2N4921,2N4238, FCX1053A
ZD1 = 5.6 V/1W
LED1
Miscellaneous
SW1 = On/off switch
1.5V*8 AA cells
Mobile connector
Source- http://electronicsproject.org/mobile-cellphone-charger/