Some ICs (usually digital logic) have open-drain outputs like this, or you can replace M1 with a discrete MOSFET or an NPN transistor. Now of course you could build the circuit with a PNP, like so: All the solenoid cares about is that there's a 12V difference between its terminals. This is because "voltage" is measuring electric potential difference, and because it's difference means it doesn't matter which point we decide to call "0V" or "ground". The choice of what to call "ground" is arbitrary, and while this particular choice is non-conventional and confusing to talk about because it violates the conventions of what "ground" is typically, it's electrically identical. Now the solenoid is connected to ground all the time, but this circuit is identical to the one in your question. Electronic components know nothing about it. "Ground" is a concept people made up to simplify discussion about a circuit. Why is that? Keep in mind the solenoid doesn't "know" which end is ground. It seems to logically make more sense to have the solenoid connected to ground all the time, then when it's time to activate the solenoid, simply pull up the solenoid using a PNP. NPNs (and their N-FET cousins) offer somewhat better current handling capability than the P-type devices, but only slightly so given modern process technology. Same goes for MOSFETs: a low-side N-FET can be switched with logic a high-side P-FET needs a level shifter if it's switching a voltage higher than the logic level.Īnd here's the high-side switch, with MOSFETs: R2 should be sized based on the required Q1 saturation current.) R3 isn't strictly necessary, it pulls the Q1 base up to +12 when Q2 is off to improve noise immunity. The shifter ensures that the PNP base is pulled up high enough to reliably turn off the device. Simulate this circuit – Schematic created using CircuitLabįor higher-than-logic voltages like the example, a high-side drive PNP needs a level shifter (such as another NPN) to translate the voltage up. Low-side drive like the NPN shown can be controlled directly by ordinary logic levels yet can manage a higher controlled voltage (like the +12V shown in the example.)Ī PNP (or P-FET) can be used to switch on the high side, if the switch supply is the same or lower than the logic level. However due to the inverse behavior of a PNP to an NPN, it would take a little more work then just having the Arduino output a HIGH signal. It seems like an NPN would be the best choice for this scenario, but I don't really understand how connecting the solenoid to ground would activate it? It seems to logically make more sense to have the solenoid connected to ground all the time, then when it's time to activate the solenoid, simply pull up the solenoid using a PNP.
After searching around online, it seems that a general schematic for that would look like this (asides from the BJT): So when the Arduino outputs a HIGH signal, the solenoid should activate, and at LOW signal, it shouldn't do anything. One question I had was if there was any advantage to using a PNP transistor over an NPN transistor? From class, I know that PNP's are usually better for pulling devices high and NPNs are better for pulling devices low, although I am unsure why this is the case.įor reference, I want to use an Arduino to control the transistor that activates the solenoid.
I am working on a circuit that controls a solenoid through the use of an Arduino.
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