LED issue with basic relay bypass

S

snow

New member
I incorporated the Basic Relay Bypass system into an overdrive/eq pedal I designed, but I'm encountering a small issue. The pedal sounds great, there's no cutting in and out, or any obvious voltage starvation, but for some reason when the pedal is on, the indicator LED is flashing instead of being static. I added a trimpot to adjust the brightness of the LED, and while it does adjust the brightness, it also adjusts the speed of the flashing (but the range is pretty small).

Circuit Schematic: tube booster schem 2025.png

PCB design:
tube booster pcb 2025.png

Voltages:
NE555
Pin 1: 0v
Pin 2: 2.54v
Pin 3: 3.67v
Pin 4: 5.10v
Pin 5: 3.39v
Pin 6: 2.54v
Pin 7: 0-0.05v (varies)
Pin 8: 5.1v

Voltage regulator (LM78L05)
Output: 5.10v
Ground: 0.0v
Input: 8.96v

Q4 (2n5088)
Emitter: 0.0v
Base: 0.65v
Collector: 0.0v-0.09v (goes up and down, and seems to vary based on trimpot position)
 
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pin 4 and 8 of the 555 should be tied to VCC, you have it tied to VREF.
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Well actually they're tied to +5v (from the voltage regulator), VREF in this case would be the 10k/10k voltage divider. I also made a similar post on diystompboxes, and I asked about that (it's been about a year and a half since i got the boards (life and school got in the way of me building it)), and nobody indicated that it would be an issue.
 
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If you're running the 555 at 5V it really can't deliver enough voltage to properly latch the relay.


Running at 5V, Pin 3, which is the pin that drives the relay, is only outputting 3.6V.

You're losing another 0.3V across D20 so your 5V relay is only getting about 3.3V.


Does the voltage on pin 3 fluctuate when the LED is blinking?
 
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Yes. With the trimpot maxed, it fluctuated from around 3.9v to about 5.8v (but at one point I think it was stable at 4.4v), but it wasn't changing in sync with the blinking.
 
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