This project was build out of necessity when building the Light Controller interface. In the early days of PIC IC's, flash memory was in its early days and PIC memory was erased by using ultra-violet light, in the same way Eprom memory ICs are erased.

The 2 main requirements for an eraser are to have a controlled timed period of operation and potection from the UV light, since the frequency of UV light required is not good for our eyes. So this eraser is enclosed with a water/light proof lid and a timer.

The control circuit for this project uses a relay to switch the UV light. There is no reason why it could not be used to time any other low current device.

WARNING: This project swithces mains electricity (220v AC). Great care should be excercised when connecting and testing this project, and only someone of adequate competance and experience should attempt it.



The heart of the timer is the astable formed around IC1 and adjusted by VR1. This IC also contains a 14-stage binary ripple counter. The astable output is fed to the ripple counter (internally) dividing its frequency by 8192 (2^14). The output is then sent to the next counter which divides it by 10 with outputs at each stage.

The selector switch SW2 is a multipole type that selects one of the dividers outputs to select the desired time period.

The power to the circuit is from a miniature 9v AC transformer. D3-6 form a bridge rectifier and C6 smooths the DC voltage to around 12.6v for the 12v relay. RG1 converts the voltage to 5v for the IC's. The IC's will accept up to 15v supply, but the regulated 5v means the astable can be set and stable.

To start the timing, SW1 is pressed, which activates the relay which powers up the circuit. C1/R1/D1 reset the counters by producing a brief positive pulse. The output from the divider section is initially low. Q1 is therefore off and so Q2 is allowed to saturate via R5 and R6, activating the relay and taking over from the switch. The circuit is now timing and the second pair of relay contacts, connected to the mains UV light are closed, turning on the UV light.

Once timing has completed and the output from the divider goes high, the transistor Q1 saturates pulling the base of Q2 low, switching of Q2 and the relay coil, thus releasing the relay contacts and turning off the circuit and the UV light.


The timing is controlled by the astable. The formula for the astable is approx f = 1 / (2.2 C R) where C is the capacitor C2 and R is the combined value of VR1 + R2.

So the minimim (Rmin) will be 47K and the maximum 147K (Rmax). Putting this into the equation we get:

Fmax = 1 / (2.2 x 47x10^-9 x 47x10^3) = 205 Hz

Fmin = 1 / (2.2 x 47x10^-9 x 147x10^3) = 65 Hz

This frequency is now fed into the 14-stage ripple counter. The Q13 output (pin 3) will give a divided down frequency. The on/off cycle time of this output will be T = (1 / F )  x 2^14 so:

Tmin = (1 / 205) x 8192 = 40 seconds

Tmax = (1 / 65) x 8192 = 126 seconds

The timings to the selector switch for 120 second setting, will be not quite as expected. They are 1, 3, 5, 7... not 2, 4, 6, 8 minutes. The reason for this is shown in the timing diagram.

Assuming Tcyc is set to 120 seconds, then each output from the divider will change every 2 minutes. The clock input to the divider will start low, then go high after one minute and then go low again after another, thus a cycle time of 2 minutes. Now the divider advances on the rising edge of the clock input, which goes high half way through a cycle, so it will advance after 1 minute then 3 then 5.


PCB Size: 102mm x 48mm

The rotary switch SW2, is a PCB mounted type. This range of switches has various pole/way configurations. This type is a 1-pole/12-way type. You can use a 2-pole/6-way by removing the selector ring around the base of the switch so that all positions are accessible.

PCB tracks

The PCB track layout should be reproduced so the board measures 102mm x 48mm. The track layout shown is viewed from the component side, as though you were looking through the board.

Photo-etching was used to reproduce the PCB, by printing the image onto transparent acetate sheet. All connections to and from the PCB are via PCB terminal blocks.


The wiring is fairly straight forward. WHen wiring the high voltage AC, make sure to use mains cable. Wires stripped from a flexible 3-core 5 amp cable are fine.

Transformers generally have a single secondary, or dual windings. In the diagram is a dual type giving a 9v-0v-9v supply. Since we only want 9v and not 18v, the center tap and eith winding was used, while the other winding was terminated in a spare block connector. With a single winding secondary, there are only 2 wires so no problem.

SW3, the power switch is a "push-on push-off" type while the start button, SW1 is a simple push-to-make type.

The PCB is meant to be mounted on the front panel of the enclosure using the thread on the rotary switch SW2. Care must be taken, since the thread is plastic and it is easy to damage the thread with the metal securing nut. Cross-threading is easy, beware.

UV_Eraser Inside

The mains components were mounted on a piece of 5mm acrylic and connected with the terminal block. The UV light was stripped from an original light box with a broken case.

The UV light was mounted in the lid but I never got around to putting in a foil reflector, thus the discolouration on the lid. The IC sits on another 3mm piece of acrylic, that fits on top of the electronics.

The entire project was fitted in an IP65 weatherproof case which was ideal for being light-proof.

UV Eraser

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