Built on the lines of payphones, here is an automatic coin collection device for payloads like lamps and air-conditioners to be used on a private electrical line. It is useful for paying guest houses, lodges and trains.

The system makes use of a sensor for detecting the coin and a microcontroller that counts the coins and shows the count on a 7-segment display. When you close the load switch provided in the circuit, the relay energises to connect the load and the coin count on display starts decrementing. When the count decrements to zero, the relay de-energises to disconnect the load.

Circuit description

Fig.1 shows the put-coin-and-draw power circuit. It comprises microcontroller AT89C2051 (IC2), dual timer NE556 (IC1), 7-segment decoder CD4511 (IC3), regulators 7805 and 806 (IC4 and IC5), and a few discrete components.

Fig.1:Put-coin-and-draw-power circuit
Fig.1:Put-coin-and-draw-power circuit

LED1 is used as the light source for light-dependent resistor LDR1, which is made of cadmium sulphide and acts as the coin detector. Resistor R1 limits the current through LED1. The light from LED1 falls continuously on LDR1, whose resistance decreases with increase in the incident light intensity.

The NE556 dual monolithic timing circuit is a highly stable controller capable of producing accurate time delays. It is basically a dual NE555. In the time delay mode of operation, the time is precisely controlled by an external resistor and capacitor. The two timers operate independently of each other, sharing only VCC and ground. The circuits may be triggered and reset on falling waveforms. One timer of NE556 is used for coin detection.

LDR1, connected at trigger pin 6 of IC1, offers low resistance when light is falling on it and its trigger input goes low to set the flip-flop and make output pin 5 of IC1 high.

Fig.2:Power supply circuit
Fig.2:Power supply circuit


When a coin is inserted, it interrupts the light falling on LDR1, and trigger pin 6 of IC1 goes high to make output pin 5 low. This high-to-low pulse is used by the microcontroller to display the coin count.

Microcontroller AT89C2051 is the heart of the circuit. It is a low-voltage, high-performance, 8-bit microcontroller that features 2 kB of Flash, 128 bytes of RAM, 15 input/output (I/O) lines, two 16-bit timers/counters, a five-vector two-level interrupt architecture, a full-duplex serial port, a precision analogue comparator, on-chip oscillator and clock circuitry. A 12MHz crystal is used for providing the basic clock frequency. All I/O pins are reset to ‘1’ as soon as RST goes high. Holding RST pin high for two machine cycles, while the oscillator is running, resets the device. Power-on reset is derived from resistor R6 and capacitor C7. Switch S1 is used for manual reset.

Coin-detection output pin 5 of NE556 is interfaced with port pin P3.0 of the microcontroller (IC2). The microcontroller program counts the number of coins inserted and the count is shown on a 7-segment display.

The ‘A’ through ‘D’ inputs of 7-segment decoder IC3 are interfaced with port pins P1.4 through P1.7 of IC2. IC3 accepts the BCD input and decodes it to show on the 7-segment display. Coin detection is also indicated by LED2, which is connected to pin P3.7 of the microcontroller.

After inserting the coin, close load switch S2. Port pin P1.1 of the microcontroller goes high to drive transistor T2 into saturation. Relay RL1 energises and LED3 glows to indicate that the load is now switched on. D1 acts as a free-wheeling diode.

As power is drawn by the load (pin P1.1 high), the count shown on the 7-segment display (DIS1) decrements. Port pin P1.0 of the microcontroller triggers the second timer of NE556. When trigger pin 8 of NE556 goes low, its output pin 9 goes high for a time period decided by preset VR1 and capacitor C4. The high output of the timer is inverted by transistor T1 and fed to port pin P3.2 of the microcontroller (pin 6 of IC2). The count display decrements by ‘1’ after port pin P3.2 of the microcontroller receives five pulses (indicated by glowing of LED4).

Fig.2 shows the power supply circuit. The 230V AC mains is stepped down by transformer X1 to deliver the secondary output of 9V, 500 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D2 through D5, filtered by capacitor C8 and then regulated by ICs 7805 (IC4) and 7806 (IC5). Capacitors C9 and C10 bypass the ripples present in the regulated 5V and 6V power supplies. LED5 acts as the power-‘on’ indicator and resistor R25 limits the current through LED5.


An actual-size, single-side PCB for the put-coin-and-draw-power circuit is shown in Fig. 3(View as PDF) and its component layout in Fig.4(View as PDF). Wire the circuit on the PCB and check for proper connections using a multimeter.

Download PCB and component layout PDFs (Fig. 3, 4): click here



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