Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Protection for white LED in torch

The article on the LED Torch published in the December 2000 issue warns against powering up the circuit without the LED and then subsequently connecting the LED. This can destroy the white LED from over-current due to the over-charged tantalum capacitor. While this should be a "remote possibility" because the LED is hard-wired into the circuit, in fact that LED-destroying condition can occur fairly easily.

The problem arises when the screw-type torch is used and the wire twists off, as it inevitably will, from screwing and unscrewing to change the battery (or what is more likely, to show other interested parties what is inside!). Then, when investigating why the LED will not light, the LED can make momentary contact again and will immediately blow from over-current.

The suggested fix limits the voltage across the tantalum capacitor with a 4.7V zener diode (which normally does nothing because the LED holds the zener below its operating voltage) and uses a 15W limiting resistor in series with the LED to ensure that it can be safely connected or disconnected and at no time exceed the 200mA rated absolute maximum peak LED current.

The 15W resistor does waste some power but this amounts to less than 10% and is well worth sacrificing to help provide a fail-safe circuit for the expensive LED.

The zener also protects the tantalum capacitor from over-voltage breakdown that can otherwise occur when the LED is not in circuit.

The total cost of the modification is less than 50 cents and the additional components can be mounted on the existing PC board.

Rick Matthews,
Adelaide, SA. ($30)

Relative field strength meter for a DMM

Many passive field strength meters have appeared in the past, typically using a 50mA analog meter movement if reasonable sensitivity was to be obtained. This circuit is similar but has the advantage that it works with the high-impedance load of a digital multimeter, typically switched to the 200mV range.

The sensitivity is adequate for low power equipment like CB radios, cordless phones and model R/C sets (cars, model airplanes, etc).

For best results, use OA81 or similar germanium diodes. Modern Schottky signal diodes could also be used but the results are not as good.

The circuit can be wired directly into a small plastic box with protruding banana posts to match the terminals on your DMM. A banana jack can also be used for the antenna which could be just a 500mm length of wire as a starting point.

Gerard La Rooy,
Christchurch, New Zealand. ($30)

Constant current LED drive Most LED driver circuits use a series resistor to control the current through the LED. For applications needing a few LEDs, this is optimal. However, for applications needing many LEDs, this becomes extravagantly inefficient and it is tempting to keep the voltage drop across the resistor as small as possible. That leads to poor control of the current. ICs such as the MM5450 and its relatives and the A6275 and its relatives provide constant current outputs so that the current through the LEDs is well controlled even though the voltage drop across the circuit doing the control is acceptably small. However, the difficulty with these circuits is that because they contain many constant current drivers crowded into a relatively small package, unless the supply voltage is small, they become too hot and can destroy themselves. This problem is not easy to solve. The solution is to maintain a small voltage across each constant current source. In this circuit, this is accomplished by REG1, the LM317L, which provides a bias of about 1.5V ±5%. Each transistor works as an emitter-follower, presenting the A6275 inputs with about 0.9V. Vled, the LED supply voltage, needs to be high enough to ensure that there will be at least 0.5V across each transistor but it is safe to allow significantly more than this and the supply need not be well regulated. The transistors can be general purpose NPN types such as BC548 and a single LM317L will easily supply a total LED current of at least 1A. A6275s are made by Allegro and are available from Farnell, catalog numbers 300-5306, 300-5318. Keith Anderson, Kingston, Tas.