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Old 11-19-2013, 11:56 AM
rjdekker rjdekker is offline
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Lightbulb Control digital stompbox switches using MPA logic ports

Hi all,

I could really use some help with midifying a digital stompbox using an MPA revB. It's an expensive box so I don't want to wreck it by wiring it incorrectly. Replacing the 10 kOhm hardware potmeters on the unit with the ones on the MPA seems straightforward. These pots control the digital circuitry using a 0-5 Volt signal. This should not be a problem I quess.

Questions I have:

1) The stompbox can be controlled by two external footswitches that internally connect the logic inputs of a PIC16F877 (RA2 and RA3) to ground via pull-up resistors outside the PIC (see Footswitches.jpg). Which of the following would be best? Connect the logic outs on the MPA directly to the logic ins on the PIC (ie. add red connections and cut at red marks) or leave the 10 kOhm resistor inbetween the MPA and PIC (add blue connections and cut at blue marks).

2) An internal relay is used for true bypass, which is controlled by a logic out (RA5) on the same PIC microcontroller. The internal circuit is shown in Bypass_relay.jpg (I hope I reverse engineered it correctly). Can I simply cut the line to the PIC logic out (leaving it unconnected) and connect one of the MPA logic outs to drive the relay circuitry?

3) The specs on the MPA state that the PSU should be between 7.5V and 12V. I intend on using 12V DC since the stompbox also runs on 12V. Is it wise to power it with 12V or should I add a heat sink to the MPA's voltage regulator? The amp rating of the PSU is more than sufficient for the unit and the MPA.

Thank you so much for any help in advance!

With kind regards,

Rob
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Last edited by rjdekker; 11-21-2013 at 07:00 PM.
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Old 11-21-2013, 02:41 PM
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John John is offline
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Hi Rob,

Quote:
Originally Posted by rjdekker View Post
Hi all,

I could really use some help with midifying a digital stompbox using an MPA revB. It's an expensive box so I don't want to wreck it by wiring it incorrectly. Replacing the 10 kOhm hardware potmeters on the unit with the ones on the MPA seems straightforward. These pots control the digital circuitry using a 0-5 Volt signal. This should not be a problem I quess.

Questions I have:

1) The stompbox can be controlled by two external footswitches that internally connect the logic inputs of a PIC16F877 (RA2 and RA3) to ground via pull-up resistors outside the PIC (see Footswitches.jpg). Which of the following would be best? Connect the logic outs on the MPA directly to the logic ins on the PIC (ie. add red connections and cut at red marks) or leave the 10 kOhm resistor inbetween the MPA and PIC (add blue connections and cut at blue marks).
I hesitate to give advice without seeing a schematic of the connections. Can you upload a sketch? (your images are not loading for me, but if you attach them to your post and they will be hosted here at the forum.)

Generally speaking, if you have a switch that connects a signal to ground, you can safely "flip the switch" using a logic output with a series diode. That prevents a high-level logic output from being short circuited to ground when the switch happens to be closed.

Quote:
2) An internal relay is used for true bypass, which is controlled by a logic out (RA5) on the same PIC microcontroller. The internal circuit is shown in Bypass_relay.jpg (I hope I reverse engineered it correctly). Can I simply cut the line to the PIC logic out (leaving it unconnected) and connect one of the MPA logic outs to drive the relay circuitry?
Probably. Be sure to check the current draw of the relay and compare to the current spec for the MPA logic outputs (in hardware user manual).

Quote:
3) The specs on the MPA state that the PSU should be between 7.5V and 12V. I intend on using 12V DC since the stompbox also runs on 12V. Is it wise to power it with 12V or should I add a heat sink to the MPA's voltage regulator? The amp rating of the PSU is more than sufficient for the unit and the MPA.
This is just a matter of checking the temperature of the voltage regulator during operation in your application. It includes a thermal shut-off, so it won't be damaged by overheating--it will just shut itself down. Add a heat sink if it gets too hot for your liking.
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Old 11-21-2013, 07:27 PM
rjdekker rjdekker is offline
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Hi John,

Thank you for your time and helpful advice.

I've managed to attach the files now:

1) The two switches (SW1 and 2) will be permanently disconnected (always left open). I intend to only "switch" using the MPA. I was assuming that disconnecting the +5V and keeping the resistor between the PIC and MPA would be sufficient, but if not...could you please recommend a specific diode (or specs)? My knowledge of electronics is limited to having a basic understanding of the workings of these circuits and the ability to hold a soldering iron.

2) Rated current for the relay is 30 mA. Maximum source/sink current for the MPA logic outputs is 25 mA. I don't know if this is compatible with the circuit in the figure I have attached?

Thanks so much again! I can't wait to start doing the modifications. It will be really helpful to control this unit by MIDI so I can automate all parameters during recording sessions.

Kind regards,

Rob
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Old 01-09-2014, 08:03 PM
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John John is offline
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Hi Rob,

Quote:
Originally Posted by rjdekker View Post
I've managed to attach the files now:

1) The two switches (SW1 and 2) will be permanently disconnected (always left open). I intend to only "switch" using the MPA. I was assuming that disconnecting the +5V and keeping the resistor between the PIC and MPA would be sufficient, but if not...could you please recommend a specific diode (or specs)? My knowledge of electronics is limited to having a basic understanding of the workings of these circuits and the ability to hold a soldering iron.
If your diagram represents the entire system, then yes, I think that what is shown will work fine.

Quote:
2) Rated current for the relay is 30 mA. Maximum source/sink current for the MPA logic outputs is 25 mA. I don't know if this is compatible with the circuit in the figure I have attached?
Yes, it looks like the circuit will draw much less than 25mA from the MPA logic output.

Sorry for the slow response!
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