Cockpitbuilders.com

Hello Captains,

I would like to ask a few questions concerning the Pokeys 56U and solenoid connection. Looking at the schematic on page 45 of the Pokeys Manual it shows the connection from  a Pokeys port and on through a 4k7 resistor and on to a npn BC547 transistor. The arrow side of the transistor goes to ground, the other side goes to  to the relay (5v). The relay is in parallel with a diode before going to the +12volt side of the relay.  (Hope this makes sense)

My question is, what does the transistor do?  I plan on using a rotary switch , so would this mean I wont need the transistor?  I'm also not sure why the use of the resistor. My relays are 5 volt so logicaly thinking..  do I need a resistor when my relays will be 5 volt?

Hope someone can clarify this for me. Thanks

(Edited twice for spelling errors)

The transistor is acting as a switch to turn on the relay. It is needed as the card likely cannot supply enough current to turn on and off the relay coil. The resistor is needed to bias the transistor to operate in the correct region so it acts as a switch.

This is a standard circuit for driving a relay from a microcontroller. I would not modify anything as that is what the card manufacturer has specified, and would be required for the controller that is on the card.

What do you mean by you plan on using a rotary switch? You use the card to drive the relay / solenoid

Hi Kyle,

Thanks for your response.  What I meant was that I will be using a 45 degree rotary switch for the engine start sequence. The rotary when switched to GRD would signal the card to activate the solenoid and return the rotary to OFF.  For some reason I was thinking the rotary switch could replace the transistor as the switch.
Thanks for the clarification concerning the resistor.

All the best
Ed