Two leds for one

circuit.jpgThat push button bistable I have still yet to get going is part of an attempt to replace an led in an existing lamp with a choice of two.

I thought I would reveal the rest of my circuit for scrutiny.

The idea is that the whole circuit connects directly into the lamp instead of the existing led, stealing a few microamps to run itself.

The pairs of diodes ensure that there is always a load on the power source, which is a dc-dc converter, even during switching.

I tried just one diode, but the Cree leds I am using seem quite happy to produce a significant amount of light, even with only 2.5V across them.

The two diodes cut this to about 2.2V when the active led is dropping 3V, leaving only a faint glow from the nominally ‘off’ one.

There are a whole bunch of mono, bi and n-stables on this blog and they were mostly hyperlinked until some IT changes at EW disconnected them all. As I can’t face putting all the links back in the right places, here is a list instead.
A 74HC74 hysteresis mystery
Hysteresis mystery solved, probably
Two leds for one
And now the pcb
Simple circuit selects LEDs? – a tri-stable
More push button bistables
A 74HC4060 monostable?
Thoughts on another EEVblog – a power on-off bistable

Tags: Alice, blog, push button

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  1. 'Alice'
    March 22, 2010 16:56

    Thanks Stuart.
    I did think about a toggle switch, what stopped me was that there was only a tiny place to mount the switch, and it had to be glove-friendly.
    As well as that, the control had to be at the end of some wires and I worried about RFI because the supply to the leds is noisy.

  2. Stuart
    February 11, 2010 12:06

    Will not a switch alone suffice?
    2 LEDS, to 2 poles of a toggle switch.
    Position 1 connect LED 1, position 2 the other LED.

  3. 'Alice'
    February 01, 2010 16:45

    Sorry, what ever we try, the comments on Movable Type seem to be picture-proof.
    I am most interested in this – I once made one with NOR gates in a ring but, although it worked, I was never quite sure how.
    You can email it to and I can stick it up in a new posting, or add it to this one.

  4. Mike Meakin
    February 01, 2010 14:34

    How do I post a picture in a comment ? I have a nifty little circuit as an alternative to the 74HC74

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