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Hello Russ and Brian, I just got a message from Scompracer that the pin he was using from his Arduino to switch the relays on the sidecar has died. On the sidecar there are already a resistor, transistor and diode present, so it should be very unlikely that to much current has been drawn from the pin at the Arduino. He also wrote to me that you (Russ) had replied to him that a transistor and fly-back diode must be used when using a Arduino to switch the relays, but these are already there. I must be missing something here. Is the transistor not used as a switch here? - Edited by user Tuesday, October 30, 2012 10:22:38 PM(UTC)
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The transistor is just a switch. the resistors is there to limit current. The diode is for flyback protection.
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I found the thread Russ mentioned using a transistor. This was in Corpius's using Arduino thread, and was (I believe) in response to using Arduino to control the Sidecar. http://www.twistedpearau...aspx?g=posts&m=14611"I would definitely use a transistor (with a fly-back diode) to drive a relay." We see there is already a transistor controlling the relays so some of us figured it would be Ok to use pin 6 of the Arduino to activate the Sidecar relay. I stuck a Fluke in series, mA range, between 5v B+ and Sidecar trigger and get higher than 90mA spike when switch closed, then it settles down to 20mA. The Arduino is rated at 40mA DC Current per I/O Pin, 50 mA for 3.3V Pin. I had switched inputs to external CD player, Sidecar clicked in, it worked and then quit. It had been working fine before. No output from pin 6 on Arduino UNO now. Code is the same. Pin 6 is defined. Just point me the way...like what transistor to use and where to connect it. I am guessing take another QN1 lik on Sidecar, attach leg 2 to VD 5v, leg 3 to trigger 2 (B). Arduino pin 6 would activate leg 1 of QN1 (through resistor like Sidecar?) which activates relays of Sidecar? Edited by user Wednesday, October 31, 2012 4:31:26 AM(UTC)
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Hi Rich, I thing that using a second transistor switch circuit is unnecessary. I just noticed that the schematics for BIII modules are available. The schematic for the sidecar shows already a transistor switch: http://www.twistedpearau...al/sidecar_schematic.pdfPerhaps it is a better idea to limit the current to the base of the transistor even more, but I do not know if this is possible. It's depending on the type of transistor and the value of the current resistor. It's quite easy to calculate the maximum resistor value that can be added without causing the switch to fail. You have to know some details about the transistor, relays and the current resistor value. The detail about the transistor and relays can be found in their datasheets Some time ago a made a excel spreadsheet that automatically calculates minimal and advised resistor values depending on the used transistor and the current draw from the circuit, which are the relays in this case. I'll try to attach it. Just subtract the current resistor value from the calculated minimal ON value and this should be the maximum resistor value that can be added without the circuit to fail. Perhaps it's better to use a some what lower value to make sure that the switch still operates. File Attachment(s):  Transistor calculator.zip (3kb) downloaded 2 time(s).You cannot view/download attachments. Try to login or register. |
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For this application, I use a ULN2803 withtout any problem.
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What value resistor are you using for R1? The supplied value is 10K, which should limit current to a few mA.
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Originally Posted by: Brian Donegan  What value resistor are you using for R1? The supplied value is 10K, which should limit current to a few mA. My bad, in the move to the dual mono build I had forgotten I replaced R1 with a 1K resistor for activation by the manual selector. I have the 10k resistor in place now so the new Arduino UNO on the way shouldn't have any problems.  |
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