I have a few questions relating to an idea one of my team members had earlier today…
We’re ordering a couple of new green 60mm LED rings which we are going to quickly put on in the pits at worlds. The standard LED rings which everyone buys break if you look at them too hard and the green is always out of stock.
Considering these have 10 times the intensity we wanted to make sure we could toggle them off after autonomous is over. I paged through the rules and found that relays (outside of the expensive and out of stock spike relay) were illegal so the only remaining option was to make a custom circuit with a MOSFET to avoid having any actuators.
Would this be legal? Has anyone tried it? Looks like we can just put a resistor and a MOSFET on some perf board and feed it a digital output from the rRIO. Thoughts?
I agree that relays that control active mechanisms (i.e. Spike relays) are expensive and no longer in production. That being said, there are no rules about what kind of relays you can use inside a custom circuit. You should be able to find a cheap(ish) relay from any local RC or hobby store that can handle the relatively-small current from the LEDs. As long as it only controls LEDs and nothing active, it should be perfectly legal. No need to mess around with MOSFETs.
Maybe I’m misreading R34-b but it looks like the Spike is the only legal relay? I haven’t spent too much time digging through the rules so it would be nice to know we could pick up a cheap relay off Amazon or something instead of using a MOSFET.
We used two MOSFET circuits to control two LEDS (one LED ring light and one LED strip) independently. The MOSFET is wired with the signal output on the PWM ports wired to the gate, the negative terminal of the LED wired to the drain, and the source/ground of the MOSFET is wired to the VRM (assuming you are powering the LED with the VRM) and the PWM port. Since the PWM DIO ports output a 3.3V high, you would want any MOSFET with a gate threshold below that.
If your circuit was driving an actuator, then yes, the Spike is the only legal relay. Since this is a custom circuit not driving an actuator, you can use pretty much anything.
R34
With the exception of servos, fans, or motors integral to sensors of COTS computing devices
permitted in R32, each actuator must be controlled by a power regulating device. The only power
regulating devices for actuators permitted on the ROBOT include:
Our team made a custom circuit using NPN transistors that regulated 12 volts from the PDP. we had it inspected by the main robot inspector at Smokey Mountain Regional and had no problem with it at all.
I didn’t think of dimming them… I like that idea a lot. Do you think putting the circuit on a perf board is the right way to go? MOSFETs are usually placed in PCBs, not sure if I should solder directly to tabs or what…?
Read the data sheet. Likely what you’re going to buy will have 3 pins - For Drain, Source, and Gate.
Pick a part that is an N-channel MOSFET, also know as an inverting MOSFET. This will allow you control high voltage with low voltage.
EDIT:
I use the term PWM because you’ll rapidly be turning on and off the LED, controlling the brightness via the duty cycle (Amount of on time over the period of the wave form). This is not to be confused with the PWM used to control the motor controllers, if you used the output of the pins meant for motor controllers you wouldn’t be very pleased.
I went to the Lone Star Central Regional as a spectator. I also spoke to some of the Robot Inspectors about just this topic. Since lights are not actuators, you can control them any way you want; MOSFET, NPN or relay. The control circuit and the LED’s would be considered a Custom Circuit.
If you just want on/off control The Phidgets solid state relay boards are super easy to use with a DIO port. Just hook up the input side to signal and ground on the DIO and put the output side in line with either the power or ground to the LED. http://www.phidgets.com/products.php?product_id=3054
The higher intensity LED’s the OP linked to draw 400 mA to over 600 mA. The Phidgets solid state relay boards are rated to 500 mA. Without knowing what switching device is used on the board, I would not recommend using it for loads over 250 mA to 300 mA just to be safe.
A MOSFET like the one below would just need a series resistor to the gate (say 10 kOhm) and a shunt resistor from the gate to the source (say 10 kOhm). The nice thing about this one is that the tab is fully insulated so it is harder to short it out.
There’s also a 2.5A Phidgets board (SSR Relay Board 2.5A - 3052_1 - Phidgets). Wired the same way, costs $5 more. I like the Phidgets boards primarily because they are extremely easy to wire up to (terminal blocks, no soldering required).
Be aware one one possible trap when PWM dimming LEDs.
A PWM dimming scheme works with the human eye because of persistence of vision - our eyes can’t respond to anything much faster than 70Hz so we see it as an average value.
Cameras are different. Depending on the camera you use and your PWM frequency it may be detected as on and off instead of dim. Current regulation or even a series resistor instead of a PWM technique would eliminate this potential problem.