I am building a circuit to control reversible latched relays with a micro controller. The original plan was to have two pins control each relay (I have two in the circuit). To close I would put Pin 1 High and Pin 2 Low and to open I would put Pin1 Low and Pin2 High (example figure one). However, I discovered after putting it all together my individual digital pins cannot source enough current to activate the coil (they produce ~ 20 mA while I need at least 30 mA, so I currently switch them with two pins right now). So I need an alternative circuit to flip the relays, but still want it to use only 2 pins per relay. My idea is below and I would like feed back as I am not familiar with working transistors.
FIGURE 1. Original plan (Needed more current).
I have power pins on my board that can source 150 mA, and I was thinking to use transistors to direct the flow of current to the transistors. The circuit I have come up with requires 4 transistors to direct the flow of power (I would control these with pins 1 and 2). The C in the middle is the relay circuit in figure 3.
FIGURE 2. 4 transistors controlled by two switches to control the direction of power flowing into relay circuit.
The relays would then have two transistors (S8050D data sheet is here http://www.mouser.com/ds/2/308/SS8050-1121782.pdf) each, which would both be controlled by a single pin. See figure 3. I believe I need two since the NPN transistor only allows current to flow Collector > Emitter. Please correct me if I'm wrong.
FIGURE 3. Relay control with transistors.
Sorry the pictures are so large and my hand drawn figures are rather crude. I hope they get the point across. Is there anything wrong with my design and is there a way that I could do it more efficiently, ie fewer transistors. Can I some how use a capacitor with my original plan to source more current?
FIGURE 1. Original plan (Needed more current).
I have power pins on my board that can source 150 mA, and I was thinking to use transistors to direct the flow of current to the transistors. The circuit I have come up with requires 4 transistors to direct the flow of power (I would control these with pins 1 and 2). The C in the middle is the relay circuit in figure 3.
FIGURE 2. 4 transistors controlled by two switches to control the direction of power flowing into relay circuit.
The relays would then have two transistors (S8050D data sheet is here http://www.mouser.com/ds/2/308/SS8050-1121782.pdf) each, which would both be controlled by a single pin. See figure 3. I believe I need two since the NPN transistor only allows current to flow Collector > Emitter. Please correct me if I'm wrong.
FIGURE 3. Relay control with transistors.
Sorry the pictures are so large and my hand drawn figures are rather crude. I hope they get the point across. Is there anything wrong with my design and is there a way that I could do it more efficiently, ie fewer transistors. Can I some how use a capacitor with my original plan to source more current?
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