80-Bus News

it since erasure followed by programming may not give a satisfactory result. It is also recommended by most manufacturers that an erased device should be allowed to cool down for a time (20 – 30 minutes) before it is reprogrammed since this improves the long-term stability of the data. A device which has inevitably been heated up in the erasure process will probably program satisfactorily but may not retain all the information for the guaranteed period.

The design of this eraser is very simple and additional features can be added very easily. Many camping and motorists’ shops sell single (12″) tube, battery operated fluorescent lights for about £5 – £6 which will operate off either 6 or 12 volts DC (I used the 12 volt type in my prototype). These include a ringing choke single transistor convertor and usually take about 1 amp. Watford Electronics (and some other component suppliers) sell 12″ UV tubes for about £9 which are a plug in replacement for the standard tube which is supplied in the light. A simple 12 volt 1.5 amp power supply can be constructed for a small sum and with the aid of a light-tight box and a microswitch to act as a safety power-off interlock, a basic but serviceable eraser is yours for less than half the price of a commercial equivalent.

The PSU need not be stabilised or heavily smoothed (1000 – 2000 uF is adequate) and it is best to use a light-tight metal box, earthed to avoid problems with static electricity, into which the PSU and modified light can be fitted. The purpose of the microswitch which is connected in the mains live lead and positioned so that it can only pass current when the box is closed is to avoid excessive doses of UV radiation damaging the operator’s eyes and it should NOT be omitted under any circumstances. Mains connections MUST also be well insulated.

Most of these camping lights have a metal chassis, and a flimsy plastic case and removable diffuser which are not needed for the eraser – and should be mounted so that the tube is about 1.5″ above the tray which will hold the EPROMS. These should be placed with their pins embedded in a piece of conductive foam or into a piece of polystyrene tile covered with aluminium foil. Either method shorts out the pins and prevents damage to the device during irradiation. The device carrier should be earthed to the case.