Full featured touch control programmable power supply

Now a day the most electronics gadgets equipped with touch control technology, because of sleek look, can operate with one touch, etc. Electronics hobbyist or who is interested in electronics design their own power supply unit using common variable IC regulator LM317 with selective rotary switch for output they need. Here, a project called full featured touch control programmable power supply is published in this site is touch control variable power supply.

Circuit Description of full feature control programmable power supply

A very simple logic is used to make the circuit full feature control programmable power supply, even a simple electronics hobbyist understand and build very easily. For rectification a very common bridge rectifier circuit is used to change alternating current (AC) to pulsating direct current (DC). The positive output from rectifier is fed to input pin i.e. pin 3 of IC1 where output voltage from output of the IC1 is selected by transistor T1 to T10 with the help of resistor R2 through R11. These transistors are made to switch on and off turn by turn instead of single pole 10 way rotary switch. Transient response of the power supply is improved by capacitor C2 and 50 Hz hum is filtered by capacitor C3.

For touch control very popular IC NE555 is used in mono-shot mood with output pulse width of slightly more than a second. Base of transistor T11 and T12 is connected to touch plate TP1 and TP2 respectively. The 50 Hz hum of body from transistor T11 is fed to pin 2 of IC2 as trigger and output from pin 3 is fed to pin 14 of IC, which is 4-bit decade counter. Pin 5 of IC3 is kept low through transistor T13 at positive transition of each clock pulse at pin 14. In this condition touching TP1 each time change output voltage increasing order. But touching TP2 make pin 5 of IC3­ switch to high or logic 1, in this condition touching TP1 each time changing output voltage decreasing order.

Short circuit protection circuit is of figure 2 is connected to output point A and B.

Here we use LEDs for indication by using voltmeter across A-B terminals or an analogue-to-digital converter which in turn drives the 7-segment display.



Resistors (all ¼-watt, ± 5% Carbon unless stated)

R1 = 150 Ω

R2 = 33 Ω

R3 = 220 Ω

R4 = 390 Ω

R5, R25, R29,R32 = 470 Ω

R6 = 560 Ω

R7 = 820 Ω

R8 = 910 Ω

R9 = 1.2 KΩ

R10 = 1.8 KΩ

R11 = 2.2 KΩ

R12 - R21 = 1 KΩ

R22 = 10 KΩ

R23, R24 = 100 KΩ

R26 = 22 KΩ

R27, R28 = 1 Ω, 1W

R30 = 5.6 KΩ

R31 = 100 Ω

R33 - R42 = 120 Ω

R43 = 1.1 KΩ

VR1 = 1 KΩ (potentiometer)


C1 = 1000µF/50V electrolytic

C2 = 47µF/50V electrolytic

C3, C4, C7 = 0.1µF ceramic

C5 = 0.33µF ceramic

C6 = 10µF/25V electrolytic

C8 = 470µF/25V electrolytic

C9, C10 = 0.0068µF ceramic


IC1 = LM317T voltage regulator

IC2 = NE555 timer

IC3 = 74190 up/down decade counter

IC4 = 7442 BCD-to-decimal decoder

IC5 = LM7805, +5V regulator

D1 – D6 = 1N4007 silicon diode

T1 – T10 = BC557 pnp transistor

T11 – T13 = BC547 npn transistor

T14 = AC188 pnp transistor

SCR1 = TRY 604

SCR2 = SMC SN102


X1 = 230V AC primary to 0-15V AC, 2amp secondary transformer

SW1 = On/off switch

SW2, SW = push-to-on switch

100mA fuse

11 LEDs



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