EVP:
Hi. I am trying to control the switches on a old musical keyboard. I had planned to put a npn transistor across the switch and use the arduino to control the base through a resistor. It didn't work. The keyboard is powered by a split rail 15v supply. One side of the switch measures
-15 volts the other ground i think. My electronics knowledge isnt that great, i had a go at trying to work out if i could use a pnp. but i think the miss match between the 0 to -15 and wanting to use 5v from my arduino mega to control the base is beyond what i can work out my self or with the internet. Any idea? basically how do i switch the with some kind of transistor aragment or something. Cheers
Often then I needed to interface logic level circuits to control external circuits of unknown or awkward voltages at unknown current flows, I have favored using simple reed relays so that there is complete isolation between the logic circuits and the circuit being controlled. Many small +5vdc reed relays can be powered directly from a arduino output pin, (but do install a reversed diode across the relay's coil terminals). Here is an example of a SIP packaged +5 vdc relay that allows for pretty dense construction if needed, only draws around 25 ma of output pin current. http://www.allelectronics.com/make-a-store/item/RLY-496/5-VDC-SPDT-REED-RELAY/1.html
Lefty