Basic Relay Bypass (non-latching) Questions

[MattG]

For the Basic Relay Bypass, I have some questions about the circuit's power supply. Looking at the schematic, if I understand correctly, the intent is to power the board directly from the DC jack (i.e., in typical PedalPCB projects, with this relay bypass, you'll run two wires from each pole of the DC jack, one to the actual effect circuit board, and one for the relay board).

That means pin3/VI of the 78L09 (i.e. voltage input) should generally see about 9VDC (likely a bit more for a fresh battery or typical modern pedal power supplies). My first observation, there is no polarity protection. All the PPCB effect boards I've seen have a 1n5817 in series with the DC input, I always assumed that was for reverse polarity protection (at the expense of a small voltage drop).

Second question: I believe that 78L09 is a 9V LDO-type regulator. If I'm not mistaken, it looks like it generally expects around 12V or more in order to output 9V. So is it actually able to output a constant 9V here, if the input is generally say <= 9.5VDC?

Then the two 10k resistors make a voltage divider, producing VREF, which should be 4.5V, if the 78L09 is putting out a constant 9V. It looks like VREF is used for the relay's coil power. But there will be some voltage drop across R3/100k and the 1n5817 diode, right? Isn't this cutting things a little close? And what if the source supply is a bit under 9V (e.g. low battery, or we put a reverse-polarity protection diode in front of pin3), won't VREF fall below 4.5V? If VREF is too low, it won't activate the relay coil.

I'm probably missing something here, but that's why I'm asking!

Thanks!

 

[MattG]

So I remembered I have a variable voltage power supply, and used it to do some testing. I probed on a recent build of mine.

I took measurements at two different PSU voltages, 9V and 6V. I measured in five different spots:

• the power input ("+") on the PCB, this comes straight from the DC input jack and should be exactly what the PSU is supplying
• ne555 pin8 - this is VCC, same as Vout from the 78L09
• ne555 pin2 - VREF, should be half of VCC
• the cathodes of D1 and D2 - these are 0v when the effect is off, but when the effect is engaged, if I understand the schematic correctly, is the coil voltage for the relay
• ne555 pin3 - "high current timer output signal" - it's not clear to me what voltage to expect here

Readings are as follows:

With PSU set to 9V:

• PCB power in: 8.87v
• ne555 pin8: 7.39v
• ne555 pin2: 3.71v
• D1/D2 cathodes: 5.63v
• ne555 pin3: 6.00v

With PSU set to 6V:

• PCB power in: 5.90v
• ne555 pin8: 4.54v
• ne555 pin2: 2.27v
• D1/D2 cathodes: 2.82
• ne555 pin3: 3.00v

So, first of all, it works at 6V supply, which I did not expect. I mean, I didn't run it for a long time at 6V, but the effect was clearly engaged, and stayed that way long enough for me to measure and take notes.

These measurements confirm my basic understanding of the 78L09 and the voltage divider. That is, there is about a 1.5v drop across the LDO, between Vin and Vout. In other words, 78L09 Vout = VCC, which is about battery/power supply voltage minus 1.5v. VREF is half of VCC, as expected. So I'm not sure what the intent of the the 78L09 is here... I'm under the impression that you use a voltage regulator when you want a consistent output voltage. But, at least for these LDO type regulators, if you want that target output voltage, you have to specify an input voltage that is higher than the target voltage.

What I didn't realize in my initial questions is that pin3 of the ne555 supplies some voltage. I'm not really sure exactly how to derive that voltage given VCC.

Regardless, with the 6V supply, I measured only 2.8v on the diode cathodes. If indeed this is the supply voltage for the coil relay, I don't understand how it's working. I read the datasheet for the A-4.5W-K, and it seems the minimum coil voltage is 3.38v (and as the part number implies, 4.5v is the ideal target voltage). Maybe my particular sample just happens to be forgiving in this regard?

Edit: Just thinking out loud, I'm not sure how much of a difference is required between VCC and VREF for this to work. But, to have more consistent and predictable voltages, something like a TL431 or LM317 could be used to generate VCC, i.e. can drop VCC to maybe 7v, to give some headroom for the supply voltage (e.g. account for low battery <9v). And then a 78L05 could be used for VREF. Or keep the circuit as-is, but use a boost converter on the supply voltage to up it to 12v.

 

[MattG]

Bump... anyone have any thoughts on this? Thanks!

 

[Bricksnbeatles]

I believe it’s because the drop from the regulator when powered by 9v is still enough for the circuit, while the regulator also allows the board to not get fried if you want to power the pedal circuit with 18v