Telemecanique beacons

Sometime ago I bought on Marktplaats (a dutch auction site like eBay) six Telemecanique beacons. They're smartly engineered. You can stack up to six different lights on top of each other. They automatically connect with the beacon below by using a build-in connector system. Because every light needs to be turned a little compared to the one below they're all individually addressable. The base has a built in screw terminal with six positions and one common. By applying power between the selected position and the common the corresponding light is turned on.

No drawbacks? Yes a major one! The light bulb in the beacons consumes a whopping 6.5 Watts! At 24V this implies over 300mA! Just for an indicator light...

 This is more than the whole application, for which I bought them, together. Yes, there is a LED replacement.... but the price tag is ridiculous. They cost €51.03 a piece !

DIY? Yes! But I wanted to keep the beacon unchanged. So I needed to upgrade the light bulb. I decided to remove the innards of the bulb and to replace it with a custom made LED bulb. To make it flexibele I wanted to use a current source and not a series resistor.

I've choosen the following current source for two reasons:

1. it is floating

2. the two transistors stabilizes each other

3. and it uses all low costs parts whivh I had readily available

D1-D4 are ordinary 1N4148, NPN=BC547B, PNP=BC557B. For R1 and R2 I've choosen 100 Ohm. gives a current of Ube/R = 0.7V/100 Ohm is 7mA per transistor. In total thus 4mA.

RL in this application is a string of 4 LEDs.

The beauty of this little circuit is the inherent self stabilising of this circuit. The NPN transistor sinks constant current hrough the reference diodes of the PNP transistor, which sources a constant current through the reference diodes of the NPN transistor which sinks a constant current through the ....etc...etc.

The final result: