Double vs Single solenoids & when to use which

Hi folks.

Trying to figure out what the functional difference between a double & single solenoid is. I’ve done some digging around previous posts and I’m finding what I perceive to be different explanations of how they work.

My team and I’s previous understanding was that a double solenoid makes it pressurized in both directions, while a single solenoid only provides pressure in one direction, meaning that some outside factor (a separate mechanism, gravity, a piston with a spring in it to force it in one direction) has to retract the piston, or it’ll just stay extended, limply.

Is that the case?

this was my reply to a similar question about a week ago, let me know if you are still confused at all after reading this :slightly_smiling_face: EDIT: the snipped image should say position not poison

The spring they are talking about is inside the solenoid (the valve), not inside the cylinder (the piston).

Single solenoids still pressurize both sides of the cylinder, but use spring return in the valve. So when the solenoid is not powered it returns to one state or the other.

Sometimes this can be unintended, unstrategic, or even dangerous. But for the most part we have been okay using single solenoids as long as we are well aware of our starting position.

If you need to alternate between starting positions, for example. Or need to end the match in a different cylinder state than the start, for example, then you need a double solenoid. The advantage of the single is in the cost, certainly being less. Also you can use more single solenoids on the PCM.

Double vs single solenoids have no relationship to dual acting or spring return cylinders (this is a common misconception). Any combination works.

Both double and single solenoids control a 5-port value with an input port, A and B outputs, and A and B exhaust ports. The valve can be in one of two positions: (A) input-A B-exhaust, or (B) input-B A-exhaust. The difference between double and single solenoids is solely how they operate electrically. A double solenoid has two solenoid coils: energizing the “A” side coil moves the valve into the A position, and energizing the “B” side coil moves the valve into the B position. Not energizing both coils has no effect. A single solenoid only has one coil: energizing this coil moves the valve into the B position, not energizing it moves the valve into the A position.

The difference becomes important when related to robot enabled state. When a robot is enabled it can either energize or not energize a coil. However, when the robot is disabled, the coil is always not energized.

Thus double solenoids should be used in situations when you want to keep a valve in its last position when the robot is disabled, or when the starting position (before the robot is enabled) can be different than the ending position (after the robot is disabled), or if you might want different starting states for different situations (e.g. sometimes you want to start in high gear, other times in low gear). In all other cases, either a double or a single solenoid will work; a single solenoid is generally preferred for other situations because it uses only 1 PCM output instead of 2, and because you never have to worry about the “initial state”–the valve position is always the same when the robot is powered off.


We use single acting/spring return solenoids for almost every cylinder, since it establishes a ‘default state’ for when the robot is disabled and uses less PCM ports. Double acting solenoids are useful when you want a cylinder to maintain its state when the robot is disabled, so if you don’t want a cylinder to actuate to its default state when your robot browns out or loses coms, consider using a double acting solenoid.


Double-solenoids should used for anything that holds the robot in place for a climb. We’ve also used them for heavy/long intakes, as a safety precaution in the pits. Often times there are students in the pits who haven’t been around the robot much, so someone saying “disabling” doesn’t trigger a “watch out” warning.

Our climbing cylinder last year was on a single solenoid because it is retracted at both the start and end of the match.

We did have more dangerous pinch points on last years robot than I would have preferred. And with all the cylinders on single solenoids, you had to be well aware to keep people away from the robot when it was enabled. In the pits, certainly, this has its challenges.

  • Single and double solenoid valves are different in how they are activated electrically, not in what they do to the air.
  • The number of ports/ways on the solenoid will tell you what flexibility they have in what they do with air.

Single solenoid valves have a “default state” driven by a spring, and an electrical signal must be applied and maintained to put them in the other state. Use them when you have a default state you always want before and after the match, or if the cylinder should be in one state ~ 90+% of the time and the other 10-%.

When no electricity is applied, double solenoid valves maintain the state they were last placed in. You apply a pulse of electricity to the A solenoid to move the valve to the one position, and a pulse of electricity to the B solenoid to move it to the other. (Do not energize both at the same time.)

3 port two way valves (whether driven by a single or double solenoid) will only supply and vent one side of a cylinder. 5 port two way valves can supply and vent both sides of a cylinder, supplying one while venting the other.

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