A Secure Remote Control for a Garage Door

This article describes how to design and deploy a high-security remote control for an automatic garage door opener. I've used this method continuously for nearly 30 years on two garage doors, each with three successive automatic garage door openers of different types. I've installed the secure remote controls in a total of 6 cars (two at a time). The secure remote has performed flawlessly in every configuration.

The Problem

Your house is locked. You can enter the house through the front door if you have your housekey. If a garage with an automatic garage door opener is an integal part of your house, you can also enter your house if you have a garage door remote control. For convenient access from your car, not very many people tape a spare housekey onto their dashboard with a sign saying, “Use this key to get into my house.” But many people do the next worst thing: They clip their garage door remote control to their car visor. No need for a sign in this case; the purpose of the remote is unambiguous. An alert thief can break into your car (or skip that step if your car is unlocked), lift the remote control, and literally be home free — your home.

A second problem with garage door remote controls is that they stop working when the remote's battery is depleted. Even though the remote control is in the car all the time, and even though the car itself has more than enough juice to power the remote control, the car's electric power is not available to the remote. Even in the worst case when the car battery is dead, the “dead” battery would probably still have enough power for the remote control … if only this power could be made available to the remote.

The Solution

Both the security problem and the depleted battery problem can be solved with the following approach:

1. Hide the remote control somewhere in the car's guts, after removing its battery.
2. Install a separate push-button on the car's dashboard to operate the remote.
3. Connect the remote control and push-button to the ignition switch wiring.

With this scheme, the remote control can be operated only when the key is in the ignition, and is turned to the “accessory” or the “on” position. Presumably, you guard your car key as much as you guard your house key.

Even if a potential thief knows about this scheme (such as by reading this article), he (most thieves are male) probably won't go to the considerable trouble of attempting to defeat it — unless, of course, he's a car thief, in which case you're in big trouble anyway. More likely, the thief won't even know you're using this scheme, and will think there's no remote in the car. The only evidence will be a mysterious custom button on the instrument panel.

Of course, you can't just hook up your garage door remote directly to your car's electrical system, for at least two reasons:

1. Standard 12-volt car electrical systems have too high a voltage for most garage door remotes. Direct connection would blow out the remote on first usage.
2. Standard 12-volt car electrical systems aren't really 12 volts. When the engine is running, they can often be 14 volts. More importantly, car electrical systems are very “noisy”, meaning that transients — temporary spikes in voltage — occur frequently for many reasons. Auto electrical systems are a hostile environment for relatively delicate electronics like radio transmitters.

One could solve both these problems using an inexpensive three-terminal voltage regulator integrated circuit, but I chose an even simpler solution: Just use a two-terminal Zener diode, together with a few resistors and capacitors. Either approach can supply the proper voltage to the remote control, providing “clean” power regardless of electrical system voltage fluctuations.

To implement this solution, you need to determine the voltage and current requirements of your remote control. The remainder of this article describes how I determined these requirements, how I designed the circuit, and how I built and installed the hardware. In the likely event that you have a different remote control, you can use the following as a guide to do something similar.

Voltage Regulator Requirements

Before we can design the voltage regulator circuit, it's necessary to know the current I_R the remote control's transmitter draws when the button is pushed, and the voltage V_R required for the remote. The current draw can be measured by pressing the transmitter button while inserting a milliammeter between a relatively new battery and the remote. This method takes into account the internal resistance of the battery during the relatively heavy current draw of the transmitter. The remote I used draws 8.5 mA.

You might think that knowing the required voltage V_R is a trivial matter: The voltage is generally displayed right on the battery. However, this nominal voltage can be misleading. For example, the CR2032 battery for my remote has a nominal voltage of 3 volts, but the CR2032 spec sheet says that the voltage can be anywhere from 3.0 to 3.4 volts. Sure enough, my CR2032 measured 3.2 volts. The spec sheet also assumes a steady-state current draw of 200 μA, but what we really want is the battery voltage during the brief 8.5 mA pulse when the remote is transmitting. This voltage will be lower because of the battery's internal resistance. My voltmeter measured the battery voltage under the 8.5 mA transmitter load as 3.07 V.

In summary, my voltage regulator circuit should supply a current of I_R = 8.5 mA at a voltage of V_R = 3.07 V. In the following discussion, substitute your own measured values of I_R and V_R.

Designing the Voltage Regulator Circuit

The first and most critical task is to select a suitable Zener diode. Ideally, the Zener voltage V_Z should equal V_R, in which case we can immediately set R₂ = 0. Then the remote will see the same voltage regardless of whether the pushbutton is pushed. That's the whole purpose of the voltage regulator.

Of course, Zener diodes aren't always available in every voltage one might want, so compromise is necessary. I settled on the 1N5226B Zener diode, which has a nominal Zener voltage of 3.3 V. The 1N5226B spec sheet says that this nominal voltage can vary from 3.135 V to 3.465 V. Since these diodes are cheap (only $0.11 each), I decided to order a whole bunch, measure them, and select one with a Zener voltage around 3.07 V. If that plan had succeeded, I wouldn't have needed R₂. But the joke was on me! It turned out that all of my diodes measured around 3.47 volts! Perhaps I could have gotten closer to what I wanted with a 1N5225B, which ranges from 2.85 V to 3.15 V. But I didn't know whether all I might order would turn out to be 2.85 V — too low for this application.

So in the end, I chose to use a 1N5226B at V_Z = 3.47 V, and include a small resistor R₂ to bring the voltage down to nearer 3.07 V during transmission. Using Ohm's law, R = V/I, I would need a resistance of around

(3.47 V – 3.07 V) / 8.5 mA = 47 Ω

I chose a 42 Ω, 0.1 W, 5% composition resistor for R₂. It is a good practice to check that our usage of R₂ is well below its maximum power rating. The power dissipated by R₂ when the push-button is pressed is P = I_R² R₂ = (8.5 mA)² 42 Ω = 3 mW, way below the device's 100 mW rating. With R₂ = 42 Ω, the voltage delivered to the transmitter while transmitting is V_R = 3.11 V — very close to the ideal of 3.07 V. Once again, if I had been able to get a Zener closer to the desired value, R₂ would not have been necessary.

The next design decision is to determine an appropriate value for R₁. There are conflicting requirements for R₁. On the one hand, R₁ should be as high as possible to limit the power dissipation. On the other hand, R₁ needs to be low enough to provide 8.5 mA of current to the transmitter, plus some minimum current through the Zener diode to stay well above the knee. Unfortunately, I didn't know at the time of design what this minimum current should be, so I made a very conservative guess of 20 mA. (I've since learned that this is a much higher current than necessary. I probably would have been fine with 2 mA.) Using 20 mA, the current I_S through R₁ needs to be at least 28.5 mA under worst-case conditions. The worst case for achieving this minimum current occurs when the source voltage V_S (the voltage in the ignition circuit) is lowest.

As noted above, auto electrical systems are nominally 12 V, but in practice will often be higher. This being the case, I chose 12 V as the lowest value for V_S for the purpose of calculating R₁. Since my estimate for worst-case I_Z is so conservative, this V_S estimate wouldn't cause a problem even if the ignition voltage dips substantially below 12 V. Once again using Ohm's law, R₁ = (V_S – V_Z)/I_S = (12 V - 3.47 V)/28.5 mA = 299 Ω. I actually chose R₁ = 330 Ω, 500 mW, 5% composition resistor.

Once again, we should check that R₁ and Z₁ dissipate well below their maximum power ratings in the worst case. The worst case occurs when V_S is maximum and the dashboard pushbutton is not being pressed. Maximum sustained V_S can confidently be taken to be 15 V. Sustained automotive electrical voltages about 15 V are referred to as overvoltage, at which one can expect major damage to many automotive electrical systems. So if your electrical system exceeds 15 V, blowing out your remote's voltage regulator will be the least of your problems. Assuming V_S of 15 V, we have I_S = (V_S – V_Z)/R₁ = (15 V – 3.47 V)/330 Ω = 35 mA, so the maximum power dissipated by R₁ is I_S² R₁ = (35 mA)² 330 Ω = 400 mW, a bit close to the 500 mW maximum rating, but still acceptable. More realistically, if V_S = 14 V, the dissipation of R₁ becomes 338 mW, giving plenty of slack.

To check the maximum dissipation of Z₁, note that I_Z = I_S when the dashboard pushbutton is not pushed, so the maximum dissipation of Z₁ is V_ZI_Z = 3.47 V x 35 mA = 121 mW, way below the 500 mW rating of the device.

The Capacitors

I'm not sure the capacitors C₁ and C₂ are really needed or useful for this application. However, at worst they do no harm. I added C₁ with the intent of preventing transient spikes from entering the remote from the car's dirty electrical system. C₁ is a relatively-beefy 10 μF tantalum capacitor rated at 10 V, 20% tolerance. Its impedance is on the order of 1 Ω or less in the KHz range (millisecond voltage spikes coming into the remote). However tantalum capacitors have substantial parasitic inductance, so C₁ is ineffective against shorter pulses. For the shorter pulses, I added C₂, a 1 μF ceramic capacitor rated at 25 V. My hope was that C₂ would prevent the transmitter's 300+ MHz signal from radiating through the D.C. wiring. But I've since learned that even ceramic capacitors have too much series inductance to be effective on nanosecond pulses. So C₂ probably does not serve a useful purpose. I'm now thinking you'd probably be OK without either capacitor.

Constructing the Voltage Regulator

One can mount all the components of the voltage regulator on a small printed-circuit breadboard about the size of a postage stamp. This breadboard should be packaged in a sturdy enclosure with the remote. Back in 1989, when I first deployed secure remote controls, each control was powered by a 9 volt alkaline battery inside the remote's case. Such batteries are larger than the breadboard that replaces it. Therefore, I simply installed the breadboard inside the original case, after drilling a hole for the external wires.

But in 2016 when I got new automatic garage door openers, the remotes were powered by a much smaller CR2032 battery. Perhaps a more skilled technician can find a way to mount the voltage regulator components inside the original case. However, I chose to remove the remote's PC board from its case, and install it in a (much) larger case, making it easy to use breadboard packaging for the voltage regulator. See photo.

In the photo, black wires are ground, white wires connect to pushbuttons, and the red wire connects to the ignition switch circuit. The resistor R₂ between boards supplies regulated power to the transmitter. The #10 screw and nut behind this resistor fastens a cable tie on the outside of the enclosure, just visible at the very top of the photo. This cable tie can be used to secure the enclosure to a convenient fixture in the car. The octagonal shape in the upper right of the remote control PC board held the CR2032 battery. On the voltage regulator breadboard, the largest electrical component is R₁. To its immediate left are Z₁, C₂, and C₁, in that order.

Installing the Secure Remote

Once the remote control with voltage regulator package has been constructed, you're ready to install it in your car or other vehicle. This involves the following three concurrent tasks:

• Install the package in a concealed spot in the car, but at a location where the transmitter will not be inhibited from radiating to the automatic garage door opener. I've found that high up underneath the dashboard has always worked fine. Secure the package to a fixed point so it can't rattle or fall down. Before doing so, it's a good idea to test your proposed location with the car at some distance from the garage.
• Install a small good-quality pushbutton within easy reach of the driver. If you've done everything else right, the pushbutton will be the least reliable element of your system. You don't want the system to fail because your cheap pushbutton stopped working. I've used the same two pushbuttons for 30 years, swapping them into three cars each. They mount in a 1/4 inch hole. See photos above. When you trade in your car, this empty hole will be the only obvious evidence of customizing. (You can even install a small bolt in the hole, to make it less obvious that something has been removed.)
• Wire the remote package to the pushbutton, and to ground and the ignition circuit, as described in detail below. Dress all wires with cable ties to make a neat installation and to prevent mechanical stress.

Connecting to Ground

It's important to make your ground connection (black wire from remote control, and one wire from push-button) to a proper ground — not to just any random piece of metal that looks convenient. Connect to a metal piece that already has a ground strap or factory-installed ground wire attached to it. Solder your ground wires to a lockwasher style terminal lug, and screw the lug to this metal piece. Another adequate ground is the ground wire on an accessory power socket.

Connecting to the Car's Ignition Circuit

This is the only task that requires some finesse, because you need to connect to a circuit that is sometimes live. But it doesn't need to be live while your making the connection! Just be sure to turn off the ignition key while wiring. Then there should be no need for the annoying and problematic step of disconnecting and reconnecting the battery.

For some cars, the easiest access to the ignition circuit is the radio fuse on your interior fuse box. Your owners manual should document the function of each fuse. If you don't have an owners manual, you can always use a voltmeter to determine which fuses become live when the key is turned to the accessory position. Connect the positive supply from your voltage regulator (red wire in my implementation) to the “downstream” side of the fuse circuit, so that the fuse will protect against shorts in the voltage regulator. I was able to do this without cutting any wires in the car's cabling system by plugging in a small tinned lug right next to the fuse.

For other cars, the easiest access to the ignition circuit is the accessory power socket wire. In this case, cut the wire at a convenient point, and then reattach it together with the red wire from your voltage regulator. Solder the wires together, and use shrinkable tubing to insulate and make a professional-looking patch. I'm sure your car dealer will frown on such a procedure if it's discovered, but by making a clean patch at least you'll look like you know what your doing.

An example of what not to do: Just twist the wires together and insulate with electrical tape. The fact that this looks ugly isn't the worst thing about it. Many people do not realize that an automobile is a very hostile environment for electronics: Heat, cold, vibration, dust, and mechanical stress will conspire over time to cause electrical (and maybe even mechanical) failure of twisted wires. This will never happen with a properly-made solder joint.

Another example of what not to do: Attach the hot wire from your package to a circuit that's always live. This defeats the main purpose of this entire project, which is to prevent opening the garage door without using the ignition key.

Postscript: In 1989 when I deployed my first secure remote controls, users who could not design their own voltage regulators had very little choice but to leave their home security to the fates. Nowadays, some late model cars have a HomeLink® or similar option allowing the driver to operate automatic garage door openers using a remote control built into the car itself. Eventually such systems will be common, and the hack described above will no longer be needed.