Parking Spot Recaller
Note: This project is still in progress
This page provides an overview of each of the design decisions that were made in the development of the device.
- Project Journal - for our documentation throughout the design
- Picture Gallery - to see all of the pictures for this project
I park my car in a parking garage. For various reasons… sometimes you just can’t remember where you parked or you remember where you parked last time… etc. Since my friend and I had already had experience developping circuits such as this. We deceided we could put one together without too much trouble and it’d be a cool device.
- Minimize cost -> device has to be able to be made for under $5.
- board size -> make the device take up as little space as possible in your pocket.
- Minimize number of components -> reduces number of spots to solder thus minimizing assembly
- Low Power Consumption -> be able to run for years on a lithium coin battery.
This circuit could be built with commercially available chips but it just ends up being cheaper to make it with a programmable microcontroller then it is to implement in hardware. Additionally, it takes up less board space and allows us to assemble them much easier.
The microcontroller acts as the display controller, input interface, and the internal eeprom is used to store the location where your car is parked once it’s been powered down.
Power Up Circuit:
We used a circuit which we adapted from Roman Black’s site: http://www.romanblack.com/self_swi.htm
This circuit allowed us to have a button which would turn the microcontroller on. Once it was on the microcontroller would use about 2-3mA which would hold the SCR on. Then, once the user had stop pressing buttons for a period of time the microcontroller would go to sleep thus reducing the current drawn by the pic to a few micro amps which isn’t enough current to hold the SCR on. So, the SCR turns off and therefore the PIC turns off because it doesn’t have a path to ground. This circuit allowed us to make a circuit which allowed the PIC to turn itself off with only two components a resistor to limit the gate current and the SCR which can be purchased in low quanities for about 50 cents.
The LED display we chose was a dual digit smd display made by kingbright. Although this display costs a little more then some through-hole display versions which can be purchased on surplus sites, it has a lower profile which allows the device to be thinner overall. The profile coincidently matches the profile of the tact switches we chose which makes casting the product in a cast resin very easy.
Also, the IV characteristics of this particular display were perfect for the voltage we had onboard. The PIC microcontroller runs off of the battery voltage minus the voltage drop of the SCR: 3V - 0.8V = 2.2V. At 2.2V the display gets the 20mA operating current which the provides just enough current to the LEDs and best of all no current limiting resistors are neccessary which meets our 2nd and 3rd design requirement!
Since the device is going to be put on a keyring which will be in peoples pocket or purse the buttons have to be resiliant to being pressed accidently. The switches we use are a recessed membrane switch which don’t protrude from the circuit so they don’t get activated by pressing against your leg or other object. Also, the legs on the switches are long enough that when put on the pcb so the legs just go through the holes the switch is at the same height as the LED display.
Designing circuits can be easy but packaging the circuit so it can be used effectively in it’s application makes things harder. In order to maintain our price point we realized that a plastic enclosure was unfeasible. We would be paying about $2 if we were lucky and then we’d have to mount the board in the enclosure with standoffs and then make cutouts in the enclosure for the display and the buttons.
Jay Renner is a great guy that works in the ECE department and helps maintain the EE labs. He had used casting resin before to pot inductors for the labs so they lasted longer. He generously gave us a bottle of casting resin and the catalyst and it just worked perfectly.
We that just started playing with the casting resin by filling up random containers with the resin. After experimenting with the resin a few times we had an idea for the amount of catalyst we needed and that it would be durable enough for our needs.
Next, we just needed to find a way to make a mold for the resin. Initially, we considered using small project enclosures but from our experiments it proved difficult to get the hardened object out of the molds. Also, the resin has to cure overnight so unless we only wanted to make one each night we’d need to acquire alot of the enclosures.
We then came up with the idea to take a piece of thick piece of plastic that we could cut out rectangles in and effectively make a mold. Using our CNC milling machine we cutout perfect rectangles in a piece of fiberglass. When the resin had hardened we could just poke it out and just touch up the sides with a dremel tool to give it a smooth edge.
Finally, drill a hole near the edge to put the keychain loop through and voila.