Introduction: UD-Alert. for a Boy With Autism

Ud-Alert, or better Undress Alert, but for what?

Our son, Scott, 13 years old, suffers from autism. He is nonverbal and he still has problems to show us when he needs to join the toilet.

Because of his limited communication, he takes his clothes off when he is in his room. This does not end, every time, very well. We always keep an eye on him but at night or in the morning hours it's not so easy.

I thought it would be useful to be promptly informed when he undresses.The challenge, as always with projects for children with special needs, is to build a safe device that will not hurt them, and that they cannot swallow anything.

The Idea is to sew a transmitter module in each of his sleep trousers. If he try's to remove the pants, the switch will be pulled and the an the receiver will beep. The whole thing will work over 433Mhz and will be driven by a Cr2032 3V battery.

It must be so small that it does not bother when Scott sleeps.

Step 1: Cad Files

Here you can see the basic Idea. The DXF-file for the receiver enclose can be downloaded here.

The Principe, is very easy. If he pulls on the pants, the micro switch is triggered, and the Attiny is sending a code over the 433Mhz module. I had a few other ideas, but this one, is the one that works.

The receiver is now installed in our bedroom, and a second one in the living room. Later on I will build a portable, pager like version. (prototype is already working).

Step 2: The First Transmitters and Receivers Prototypes

The first transmitter was based on an Digistump module. I used 2 different receivers, one with an 16*2 LCD display, so I could also analyze the transmission codes, and a portable receiver with a build in charger. Due to a lag of time, I've built the one without a Battery, but I will probably come back to this one.

Step 3: The Transmitter

The whole thing is now a poor man's flexible PCB for wearables :)


  • 1x 433Mhz transmitter module
  • 1x Attiny85
  • 1x micro switch
  • 1x 100k resistor
  • 1x CR2032

The Attiny is soldered on a Dip8 PCB adapter, For the micro switch, I used a piece of PVC to build a protection, so it can not be squeezed between the foam. The elastic band is attached to the micro switch to trigger everything.

All is glued on a small piece of a plastic folder. A little bit of foam and duct tape.

A piece of fabrics is used and formed to a 25cm tunnel. The transmitter is then inserted in and glued on one side, and the elastic band is attached on the other side.

If the whole thing is lying on a smooth surface, the tension on the elastic band only has to be so high that the switch does not trip

Now everything is sewn to an old trousers. I still need to find a way to make it removable for us, but not for my Son.

The software of the transmitter works now on interrupt. On interrupt it transmits two times the code, and goes to sleep, until the next interrupt. The circuit takes in standby 0.14uA. I can live with that and the CR2031 should last for 10 years in standby. Transmitting takes about 0,7mA for a very short period.

The Battery is not removable, but the circuit will work for 3 to 4 years in normal use (10 transmissions per day)

The Software can be found here

Step 4: The Receiver

It's USB powered and made out of 3mm MDF and acrylic.

The parts used

  • 1x 433Mhz receiver modul
  • 1x Arduino Mini
  • 3x Led 3mm
  • 1x piezo
  • 4x 150 Ohm resistor
  • 1x button
  • 1 Micro USB breakout board

The internal antenna did not work to receive the signal in the whole house, so I added a 20cm long wire, as an external antenna and a plastic tube to put it inside.

The look is special now, but I'm an inventor and not a designer :)

The Software can be found here

Step 5: 433Mhz

Why did I use 433Mhz?

The modules are very cheap and everywhere available.

Because of the low frequency, they work very well in a house. I had no problems in receiving the signal in and around our house.

They can look different, but they are working all on the same AM-transmission technique. I used this model, because there were no coils on the PCB, that could have been squeezed.

The 2 antennas you can see in the picture were not used at the end. I used 2x 20cm 0,6mm wire.

Step 6: It Works!!!!!

As mentioned in the title, "Yeah, it works".

So what are the first Results:Each time the pants were removed, the system informed us. That was the main goal

It's in use now for over 5 Months, and it's a really great help.

It worked in a better way, than I thought. First of all: No more room cleaning at 3 o'clock in the morning. But wait, there is more.

Because our son can not open the door at night or in the morning, the system works also as an alarm clock for us. We now know at what time he wakes up, and when he want to go to the bathroom. We have now a better time schedule for him, so that we are now faster than the Undress Alarm :).

And because of all this, Scott now starts to use the toilet more frequently on his own during the day.

I think in a year, we do not need anymore this belt, but It was a great help for him and for us, and I hope it can help other people too.



Step 7: To Do

This model is not very practical, because before washing the pants it has to be removed, by cutting through the seams and then sewing it on again.

I have a few Ideas in my head, but to find the best one is not easy. Either I will make the electronic waterproof, so it can stay on the pants, or I have to make it removable. Removable is not so easy, because I should be able to remove it, but not my son, and it shouldn't interfere with sleep.

For the moment, we have 5 working, jogging pants, and on the Weekend I remove and sew the modules on the pants. It's better than nothing.

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