Pressure Sensor Alarm Clock
Attempting to make my own, improved alarm clockBy Megan Tung
During my sophomore year of high school, I took a semester-long electrical engineering course. In class, I learned how to solder and complete a handful of easy electronics tasks. As the semester progressed the tasks became much more challenging, and the projects turned into building a light that mimicked a turn signal on a car and a solar light that charged during the day and lit our teacher's driveway at night. Both of these projects involved electrical engineering skills to get the light to work properly as well as architecture/design skills to form the casing that surrounded and protected the light, protoboard, and other electrical parts.
My final project was to create something for the Maker Faire that included everything we had learned. My partner and I decided that we wanted to make something that would be useful to high school students. We brainstormed on some of the struggles we face on a daily basis and came up with the idea of an alarm clock that could only be turned off by stepping onto a pressure sensor. The idea originated when one of our classmates came in late claiming he snoozed his alarm too many times. Our pressure-sensor alarm clock would force students to physically get out of bed and step onto the sensor to get the dreaded sound to stop.
To begin, we ordered two different types of clocks: one analog alarm clock and one digital alarm clock. I began working on the analog one, while my partner began on the digital.
It took almost an hour, but I carefully took my clock apart. My clock provided many challenges from the start. It was made of cheap plastic parts, the gears got lost, broke, and I could not get them to perfectly line back up.
Part of the inside of the analog alarm clock
I found where metal touched metal to set the alarm off and brainstormed with my partner and teacher on how to alter the system to incorporate the pressure sensor. I had a fairly simple idea of what to do. It was only a few components: a pressure sensor, NPN transistor, 1K resistor and a switch. What could go wrong?
A pressure sensor lowers the resistance when the applied force increases. Without any pressure applied to the sensor the resistance reaches 1000k ohms. In the alarm clock, I used the NPN transistor as a switch that takes a small amount of current (input) and switches it to a large amount of current (output). A transistor has a collector, base, and emitter pin. The collector pin connects to the switch, the base connects to the pressure sensor and a 1K resistor (which connects to ground), and the emitter pin connects to the power source. When the voltage of the base surpasses 0.7V the alarm turns on; when less than 0.7V the alarm switches off. Finally, the switch simply cuts off the power in the circuit. Therefore, when pressure is applied, the resistance decreases, the voltage increases, and the switch cuts off the circuit turning the sound off.
My clock involved a metal flap that when pushed down would touch another piece of metal triggering the alarm. When I attached one side of the circuit to the bottom piece of metal everything went as planned. I applied the soldering iron to melt the metal and placed one side of the circuit in. I attempted to solder the metal flap and the other side of the circuit together, but the solder did not stick to the metal flap. I tried multiple times, but each time failed. My teacher came to the conclusion that it most likely did not stick because of the metal flap's aluminum material.
Mistake after mistake, ultimately made this clock unusable. During the soldering process, I melted the side of the clock, which made it hard to close. I also realized that the teeth on the gears got melted, causing it to get stuck. After about a week of trying to get the clock to work, I decided to throw in the towel and join my partner on the digital side.
My partner successfully located what triggered her alarm, and we were able to implement the same process I was attempting with my clock. We were able to get the pressure sensor to turn the alarm off. Unfortunately, we did not realize we had placed the circuit through the sound of the alarm clock rather than through the on/off switch.
Part of the inside of the analog alarm clock
We attempted to re-attach the circuit through the switch but could not complete it because of the clock's manufacturing. This circuit error also caused the snooze button to break. Luckily the snooze button was just connecting two metal pieces, so to prevent the metals from touching, we simply put tape between them as a quick fix.
We also needed to build a prototype of the clock's stand. It consisted of two flat, wooden planks on the bottom (one smaller than the other) and a vertical wooden post that led to another horizontal wooden piece on top that held the clock. The structure looked similar to a capital "I", with the pressure sensor on the bottom, the clock on the top, and wires down the back of the vertical post. Realistically, we wanted something that pleased the eye more and was more stable, but we did not have the time to order the materials needed.
The alarm clock prior to soldering to the circuit board
We got our project working and were able to present to our peers at our schools own "Maker Fest". It gave us the opportunity to discover any issues that arose with our product before our reveal at the Bay Area's Maker Faire. We had a week to fix any issues.
Nevertheless, the day finally came where we had our project on display. Although the pressure-sensor alarm clock had its imperfections, many onlookers were fairly impressed. The only minor problem during the event was that the sensor lost some of its sensitivity.
It takes time and patience to get something ‘perfect’. If I had to do it all over again, I definitely would have gotten a higher quality clock, bought a bigger pressure sensor, improved the stand, and placed the circuit in the on/off switch. Challenges arose, but my partner and I worked together and didn't give up!
The image above is the final project we created after a month of hard work. In the image the pressure sensor is above the wood just to show its location, but when in use it was placed below the small piece of wood.
Megan Tung is an intern at Jameco Electronics. Megan is a rising senior at Menlo High School. Her interests include photography, music, and engineering.