SOLVED Basic Relay Bypass

[Big Monk]

I wanted to incorporate relay switching into my Monk Face design.

I chose the Basic Relay Bypass schematic:



I incorporated it directly, making a few changes:



Q1 is a BC547A because that's what I had on hand. Used a 78L05 and adjusted the VRef divider resistors. Added a 100u cap after the regulator.

I soldered in just the components for the switching onto a Monk Face board to test the LED first.

Here is what I am getting:

All solder connections are good. Continuity where required. Voltages seem bang on. Here's what I'm using:

https://www.taydaelectronics.com/datasheets/A-176.pdf

Looks like the dropout voltage is 1.7v.

I'm only using the 4.5v Vref for the relay and it's associated circuitry, i.e. the NE555.

Just checked voltages:

Vref = 4.52v
Vcc = 8.33v

Reg3 = 5.04v
Reg2 = 0v
Reg1 = 8.33v

NE555:

Pin 2/6 = 4.52v
Pin 4/8 = 8.33v

Switch Pin 1 = 4.52v

BC547A (2N3904 sub) collector has continuity with LED -.

 

[Big Monk]

Friendly bump.

 

[benny_profane]

Did you account for the pinout difference between the 2n3904 and the BC547A?

 

[Big Monk]

Did you account for the pinout difference between the 2n3904 and the BC547A?

Yes. My footprint for the BC series is CBE in DipTrace.

 

[benny_profane]

Are you switching bypass/engaged states? Is the issue isolated to LED behavior?

 

[Big Monk]

Are you switching bypass/engaged states? Is the issue isolated to LED behavior?

Not 100% sure. I stopped before I hooked up I/O after I saw the LED issue.

What I will do is use one of the buffer boards (I have to scrap the Monk Face and Monk Buffer boards I have due to a width issue and using a new ground spring), which is a simpler affair, and set it up completely with I/O to test.

Hypothetically speaking: If it was NOT isolated to the LED behavior, what information would we glean moving forward? Bad relay?

 

[benny_profane]

Not 100% sure. I stopped before I hooked up I/O after I saw the LED issue.

What I will do is use one of the buffer boards (I have to scrap the Monk Face and Monk Buffer boards I have due to a width issue and using a new ground spring), which is a simpler affair, and set it up completely with I/O to test.

Hypothetically speaking: If it was NOT isolated to the LED behavior, what information would we glean moving forward? Bad relay?

It would zero in on if the issue is with the transistor switch or the entirety of the NE555 switching. More information is helpful. I'd suggest removing C1 and C4 to get to get closer to the verified circuit block you're pulling from, then add components.

I must say, it feels weird to have this circuit obscured on a DIY site with DIY support. I understand that this is a part of a commercial venture, but DIY is a two-way street of information exchange. For example, the seemingly floating cap in the other thread confuses efforts.

 

[Big Monk]

It would zero in on if the issue is with the transistor switch or the entirety of the NE555 switching. More information is helpful. I'd suggest removing C1 and C4 to get to get closer to the verified circuit block you're pulling from, then add components.

I thought about removing those but the voltages being good seemed to quell my concerns. I could easily leave them out tonight.

I must say, it feels weird to have this circuit obscured on a DIY site with DIY support. I understand that this is a part of a commercial venture, but DIY is a two-way street of information exchange. For example, the seemingly floating cap in the other thread confuses efforts.

It's not intentional, I simply didn't want to confuse matters with the rest of the circuit when it's the switching in question. If I thought the main circuit played a role, I'd have included it.

Here is my switching implementation in full:

 

[benny_profane]

You may want to consider another cap after the 4.5V node to ground. But yeah, I think some more details are needed.

 

[Big Monk]

You may want to consider another cap after the 4.5V node to ground.

I could probably just shift C3 and C4 after the divider.

But yeah, I think some more details are needed.

I'll setup a full Buffer board, with I/O, tonight to check whether bypass is functional.

 

[benny_profane]

My guess is that the NE555 is experiencing an undervoltage condition.

 

[Big Monk]

My guess is that the NE555 is experiencing an undervoltage condition.

Vcc or VRef?

Vref = 4.52v but I'm taking Vcc off the cathode end of the protection diode so it's a bit under 9v.

 

[Big Monk]

I have not had a chance to wire up one of the throwaway buffer boards yet but I double checked that the NE555 I got from Tayda was standard pin out and it is.

It’s a TI NE555P.

 

[Big Monk]

So I wired up the buffer this morning. Same issues, all the same correct voltages, etc.

@Robert chimed in and said to swap to a 9v reg and change the divider. That worked for the LED issue.

Now that I had the pedal hooked up, I tested switching.

It does not appear to be toggling bypass/engaged. Volume control on the buffer doesn't function, nor do the other controls:

 

[Big Monk]

I think I may have my relay reverse.

I think my pattern was made off the bottom view from the datasheet. So Pin 1 is actually grounded.

 

[szukalski]

I think I may have my relay reverse.

I think my pattern was made off the bottom view from the datasheet. So Pin 1 is actually grounded.

Been there . You can place the relay on the other side of the board and it’ll work.

 

[Big Monk]

Yup. Dingbat of the Year award for sure.

Two issues solved:

1.) As @Robert pointed out:

Have you tried deriving the 4.5V from the 9V supply using a pair of 10K resistors like in the original schematic?

The voltage on the threshold and trigger pins fluctuates and might be too stiff with the 1K1 there.

That solved the LED issue.

2.) I had my footprint in Diptrace set to the bottom view for the relay.

 

[benny_profane]

Sorry for the delay. Point 1 above was what I was trying to get at with my previous comment.

 

[Big Monk]

Sorry for the delay. Point 1 above was what I was trying to get at with my previous comment.

I appreciate your help.

 

[Big Monk]

Been there . You can place the relay on the other side of the board and it’ll work.

Yup! That's what I did to verify operation and what I'll do on the 2 boards I saved for test units.

Also, thank you for the heads up on that other 8 Pin relay from Fujitsu. Not only is is smaller, but cheaper as well. Even accounting for shipping outside of the Chinese Mainland, they are $0.85 per unit and much less at volume.