I tried to activate two relais with one digital output pin on an arduino uno ... now I learned, that one output pin can't provide enough current to switch (both) relais.
Instead i tried switching both via transistors, while each relay has its own, their base pins are connected parallel on the same digital output pin. I soldered a circuit board. See the drawing below. Am I to stupid for electronics or just to stupid to solder ... in the end it didn't work ... I don't find my mistake, maybe you can help me to learn from my mistakes ... (I then realized that there are boards already existing serving the intended purpose and i simply purchased one ... but still i want to know where I did wrong.)
With best regards, Jan
What is the relay coil current? Which transistor?
And voltage. Using Arduino as a power supply?
Each transistor base shoukd be wired to the output pin with its own resistor.
There should be a diode in parallel with each relay coil.
See this thread, the first post has a schematic for one relay, make another copy of the same and connect it also to the same digital output.
HTH
a7
Use a separate resistor for each transistor.
Do not put the transistor bases directly in parallel because slight differences in their forward voltages will result in one of them not getting the current it needs for saturation.
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Also, when we drive a load like a relay with a BJT we assume that the hFE (DC current gain) is 10 so the transistor is fully saturated.
I would have connected the two relays in parallel and used ONE transistor.
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