Nuts and Volts - August 2015

Iknow, I know ... a tennis ball suspended from the ceiling at just the right spot is the classic solution, but with more than one person using the garage — possibly driving different cars or just backing in — that just wouldn’t cut it. Also, it would be in the way for somebody walking through the empty garage.

Some of the commercial garage parking aids that I reviewed also had their drawbacks: One system used a laser beam shining down onto the dashboard of the car; others had only a limited range of adjustment or warning lights that were too dim. The situation seemed ripe for a custom electronic solution.

The System’s Approach

I’m always attracted to projects that involve firmware as well as hardware development, and it seemed that a microcontroller — plus some way to sense the location of a car — would be a good start. As feedback to the driver of the car, I envisioned a series of bright lamps mounted on the rear garage wall which would come on as the car passed through three “zones:” 1) when the car was nearly inside the garage; 2) when it was in the prime parking area — which I think of as the “comfort zone;” and 3) when it had gone too far and was close to hitting the wall.

This job is simple enough that almost any microcontroller would do. I have used the Propeller multicore processor by Parallax ( www.parallax.com) for a few projects, and a survey of my supply cabinets turned up an unused Propeller USB project board. This board features — in addition to the Propeller microcontroller unit (MCU) — voltage regulators, clock crystal, USB interface, and a good-sized area for additional components. It would be ideal for this application.

I began to kick around ideas to sense the distance to a car. I considered a motion detector or a diode laser beam reflected off the front of the oncoming vehicle. Neither approach seemed accurate enough, and the laser spot idea wouldn’t work if you backed the car in. I could also anticipate setup problems with these schemes.

Eventually, I settled on a non-contact approach using an ultrasonic range finder to measure distance. Parallax sells these devices, good for distances up to three meters (about 10 feet), so I sent for one.

Next, my thoughts turned to the signal lamps. These had to be bright enough to be easily seen in broad daylight. I stewed about this for a time, considering and rejecting several ideas; for instance, using relays to control incandescent bulbs. I found my answer in a local automobile parts store. For just a few dollars each, I was able to pick up some running lights — also knows as clearance lights — of the type used on big truck trailers.

These lamps contain several high-intensity LEDs; are very visible — even in bright sunshine; and run nicely on 9–16 volts DC. The lamps come in two colors (red and amber), so I bought one of each and decided I would use firmware to show when the driver had strayed into the third zone (mentioned above) and needed to back up.

Refining the Idea

I didn’t want the system to run 24 hours a day — mainly as a power conservation measure — but also to turn off the ultrasonic pinger when it wasn’t needed, since bats, moths, and (I suppose) other creatures are sensitive to these frequencies. So, I looked for some way to apply power to the system when the garage door opened, leave it on for a few minutes while the car was being parked, and then automatically shut down.

A micro switch activated by the garage door opener was the most direct approach. However, there were some mechanical difficulties involved in my case, so I began looking for another plan. Like most overhead garage door systems, mine features an overhead lamp which turns on when the door opens and remains on for about five minutes. Just what I wanted!

To avoid any modification or direct connection to the existing door opener (and possible grounding and safety problems), I decided to go with an inexpensive cadmium sulphide (CdS) light-sensitive resistor which I would locate inside the overhead light fixture, close to the bulb. The change in resistance when the overhead light came on would turn on a MOSFET which would apply power to the MCU board and the signal lights.

Now, I felt I had the basic system design pinned down. With the huge capability inherent in the Propeller and the ease of programming it, any reasonable refinements could come later.

A Few Preliminary Tests

My first priority was to see if the components I had in mind were adequate for the job.

First, I checked the current requirements for the LED lamps and found them to be fairly modest — only about 40 milliamps at 12 VDC for each lamp. Adding in 30 milliamps for the ultrasonic range sensor and 20 mA for the Propeller microcontroller board meant that the whole thing could easily be powered from a small wall wart power supply.

Next, I soldered about 30 feet of twisted pair to the terminals of the CdS photoresistor and mounted it inside a clear plastic bottle. Refer to Figure 1. I fastened it with tape inside the garage door opener overhead lamp close to the bulb, and hooked up my digital multimeter to measure the resistance. I was pleased to see that with the