Ticking Turn Signals

My topic for this week’s edition of Kibbles was inspired by a video I saw (but didn’t watch) on YouTube. The title of the video was “Why do turn signals click?” I didn’t watch the video because I know the answer, and I think it’s interesting enough to write about here. It also ties in many of the concepts I’ve already written about.

As you know, when you activate the turn signal in your car it makes a clicking sound as it turns on and off. That sound is very distinct and comes from a device called a relay. A relay is a mechanical switch that is electronically controlled. The turn signal circuit in your car is actually the combination of two circuits. The first is whatever controller is being used to create the on/off pulse for the light. The second is the actual circuit for the light itself.

There are all kinds of circuits that can create an on/off pulse that would be suitable for a turn signal. With digital circuitry this is a trivial task. There is actually a digital IC (integrated circuit) called the 555. First produced in 1971, it was estimated that over 1 billion were manufactured every year as of 2003. It’s no surprise then that the 555 is the most popular integrated circuit ever made. Primarily a timer IC, it can be used in three modes: bistable mode, monostable mode, and astable mode. The bistable and monostable modes are very interesting, but not relevant to the topic here. The astable mode is what we would use to create a series of pulses suitable for driving a flashing light in our car.

There’s a problem though. Most electronics in a car run off of the nominal 12VDC from the battery (or alternator if the car is running). Running lights in cars (headlights, brake lights, turn signals, etc) are typically xenon or halogen bulbs (though LEDs are becoming more common these days). The headlights in my car for example are H7-type xenon bulbs rated at 55 watts each. Turn signal bulbs will use less wattage, but something like 20 watts would still be typical. Let’s do the math on this to find out how much current we need to push through these bulbs. The 55-watt headlight will need about 4.6 amps (Power = Current x Volts) and the 20-watt turn signal will require about 1.6 amps. Why is this a problem?

Integrated circuits, as well as most digital circuits, are great at running logical tasks, but they do so at very low power ratings. Milliamps and fractions of a watt are typical. If you tried to use their output directly to drive a 20-watt bulb, it would fry the entire IC. This is where the relay comes in. As I said, a relay is just a mechanical switch that is electronically controlled. Inside of a relay there is usually some kind of electromagnet or solenoid. When a relatively small amount of power is applied to the electromagnet or solenoid, it pulls a piece of metal onto the contacts in the switch thereby completing the circuit. The power passing through the relay can be orders of magnitude greater than the power used to engage the relay. Critically, the circuit being switched by the relay is electrically isolated from the circuit being used to engage the solenoid or electromagnet. So instead of driving the light circuit with our 555 IC directly, we would drive a relay. Because a relay is a mechanical device, when it engages and disengages, you hear an audible clicking sound. If you ever see a car with signals that seem to be flashing too quickly, this is usually the result of a bulb failure elsewhere in the circuit or a failure of the relay. The circuit is designed to operate with a specific expected resistance, and any change to this can affect the operation of the circuit.

You might be wondering about LED lights. They take a lot less power than your standard filament-based bulb. Could they be driven by an IC directly? In some very specific cases, it might be possible, but an LED’s power rating is still much, much higher than that of a tiny little IC. In order to do it, you would need to reinforce the IC and lower the LED power rating so that they could meet, but this adds lots of cost. It’s far, far easier to continue to use the relay to drive the LED.

One last thing…even when the controller for a high-powered circuit isn’t digital, we still often use relays. In fact, your headlights on your car are driven by relays. You don’t hear the click because the relay itself is typically located inside the engine compartment fusebox. Automatically switched headlights aside, why do we use a relay if the mechanical switch is being activated by our hand? It’s a safety and cost issue. If the switch by the steering wheel was directly driving the headlights, the wiring all the way from the switch to the headlights would have to be sufficient to handle that power (55 watts in the case of my car). It also presents a hazard if the switch fails right near your hand like that. Instead we use cheap, low-current components and a low-current circuit to switch the relay on and off. Then the relay activates and deactivates the headlights. This reduces the amount of high-power wiring we need to use (which is more expensive).

Hopefully this explanation of a common thing we encounter was interesting and informative. If you have any questions about similar everyday things, send me an email.