# Creating a timer in an RC circuit

In the book "Electronics For Dummies" there is a paragraph called "Creating a timer".

If you’ve got a buzzer that requires a voltage of 6 volts in order to sound off, and you’re using a 9-volt battery to power your little scare circuit, you can build an RC circuit like the one in Figure 4-5 and use the capacitor voltage to trigger the buzzer. The idea is to charge the capacitor to about 6 volts in the time you want your flatmate to think about whether she wants a beer or a lager and reach out for one, but then blast her with the buzzer.

The idea is that by choosing a time and knowing the capacitor value we can calculate the resistor value so that the capacitor charges up to 6 volts in the chosen time from the equation 2/3T = 1RC (2/3T because 6V/9V = 2/3).

Then as the buzzer requires 6V to trigger it should alarm after the specified time.

How should I connect the buzzer?

You connect the buzzer in parallel with C.

The problem is that most buzzers will conduct some current as C is charging up and this will affect your timing. I think you have to assume one of two things from the vagueness of the question and both give the same result.

simulate this circuit – Schematic created using CircuitLab

Figure 1.

• Figure 1a has a "magic buzzer" which doesn't load C until 2/3 supply is reached. It then buzzes. It may buzz until C discharges to some lower voltage. Alternatively, it may be have a low enough current that R is able to power it indefinitely so that Vc remains constant or even increases slowly.
• Figure 1b has a buzzer with some active electronics. It is powered from the supply and when the threshold exceeds a preset level the buzzer will sound. The threshold input could be made very high impedance so it wouldn't load the capacitor significantly.

The question is a little unfair. The non-thinker might be able to answer it straight-away whereas the thinker could get caught in the details.

There are so many things wrong with the quoted paragraph that it is difficult to know where to begin.

Let's start with the buzzer. There are two basic types that can make a noise when powered by DC: electromechanical and piezo. I have never seen any flavor of either type that has a threshold detector built in so it is completely off until the input voltage reaches a certain level, then comes on full blast. From the 1960's to today, none. All such devices have a minimum operating voltage below which there is no sound, but that is not what he is describing.

As above, the buzzer will start sounding quietly way before the capacitor voltage reaches the magical 6 V. And as above, the buzzer's impedance and current draw will seriously affect the R-C time delay. Not mentioned above is that even if that were not the case, the equation in the article is incorrect. Not by a lot, but it is a clear indication of the writer's command of this topic.

Also not covered is how much the buzzer volume will be decreased by having a resistor in series with it. For a multi-second delay, the resistor value will be so high that the buzzer might never sound.

I guess what is missing in the book (which I read too) is the capacitor will release it charge quickly after it is fully charged. The circuit in series will have a quiescence at the beginning as during the charging process, the capacitor acts like an open switch, but after it is fully charged (presumbly the voltage of the battery has become rather low, at least lower than 6V), the buzzer is triggered by this capacitor during its discharging process.