LED indicator latch for circuit breaks - break/LED on - break/LED off

[fritzidachl]

Hi all! I have a 3 volt (low current) circuit which momentarily breaks and is immediately reconnected. This happens repeatedly and every time the circuit is briefly broken, I need an LED to switch on (steady state) and then, at the next circuit-break, off again (steady state). I understand I need some kind of flip-flop/latch circuit but I don't know where to start - I am just getting into this stuff...
Please can anyone suggest a circuit? Thanks.

 

[MikeMl]

What will power the LED, the 3V signal, or do you have another voltage supply available? If so, what voltage?

How much current could be drawn from the 3V signal?

Can you raise the voltage on the signal line to say 5V?

 

[fritzidachl]

I was hoping the 3V signal could power the LED (it's for a guitar foot pedal that toggles a music effect via a jack lead - I can't change the voltage of the effect) There seems to be no current drawn by the pedal itself (all it does is momentarily break the constant 3V signal to tell the effect to toggle) but there is no reason I couldn't put a battery (e.g. 9v block) inside if necessary... Thanks )

 

[MikeMl]

I would still like to be sure that if you try to draw say 20mA (normal LED operating current) from the 3V signal, that you still have 3V.

Can you measure that?

Connect a 39Ω resistor to the anode of a RED LED. Connect the LED cathode to the ground side of the 3V signal. Connect the resistor to the signal, and measure the voltage across the resistor-LED combo, both with and without it connected.

 

[fritzidachl]

Thanks - I will try to check tomorrow if I can find a suitable resistor (already midnight here in Germany). I'll be in touch...

 

[fritzidachl]

After a day of electronic cannibalism I can say that a dim red LED + 40 Ohm resistor causes a voltage drop from 3V to 1.8V - this drop triggers the effect (=bad).
I guess a battery - powered circuit..?
Cheers,
Matt

 

[MikeMl]

Yes, sounds like you want to sense the 3V signal, and use that to toggle a separately-powered flip-flop type circuit, which changes state on each interruption of signal. This could be done using a CMOS flip-flop, and would run for years even on a tiny battery. If you add an LED indicator, that becomes the pacing item for how big a battery you would need. Using a Flashing LED would reduce the power drain somewhat.

Question: what does the output of the flip-flop control besides the LED? Some sort of audio switching? How much power does that take?

 

[MikeMl]

I just thought of a problem. I'm guessing that you use the foot-pedal to momentarily short the 3V signal line to ground (0V) to toggle the effect. There is already a flip-flop inside the box that switches the effect ON/OFF. Now you add an external flip-flop that lights an LED. How do you guarantee that the external flip-flop is in step with the internal flip-flop????

I'm thinking that you should just modify the "effects box" by adding an LED to it which is driven from the existing flip-flop.

 

[fritzidachl]

Aha! My whole problem is that the "effects box" (I don't want to can-open) already has LEDs BUT it is hidden back of stage. The pedal/switch to trigger it is front of stage connected by a long jack cable. In confusion at gigs, I need to be sure if the effect is on or off at any given time (before the audience hears it!)
Sadly, a really big mirror would be a bit uncool )

I'm okay with an "out-of-step" pedal LED, as long as I know its starting state - my brain (hopefully) can figure out the rest. It just needs to change state with every pedal-switch.

Does it still sound like a CMOS solution would still work? Even months of battery life would be great - a 9 Volt block would easily fit inside the pedal.

Thanks,
Matt

 

[MikeMl]

How about attaching a photo transistor (Duct Tape? [Gaffer Tape?, see I know the lingo]) to the effects box which stares into the existing LED. Use the amplified photo transistor output to light a remote LED.

 

[fritzidachl]

Elegant solution and perhaps most simple! Could also be used in other situations. Wireless with 10 metres range would be okay. I would just have to make sure the frequency didn't interfere with the other wireless systems. Any suggestions?
I suppose I could also run a second cable next to the jack lead but more leads = more mess to trip over
How easy would it be to build?
Cheers,
Matt

 

[MikeMl]

The amplified photo transistor is a piece of cake. The wireless thing makes it much more complicated, two boxes, two batteries, etc, etc.

 

[fritzidachl]

Actually, solid "cake" sounds good (and less RF interference) - I can bind the cables together with good 'ol Duck tape
So... please can you advise me on an "amplified photo transistor" circuit?
Thanks,
Matt

 

[fritzidachl]

It's basically the "big mirror" I was joking about, isn't it?
Low-tech is the best