Column: Circuit drill
Performance of a low-power alarm circuit with an unstable power supply
By Dr. Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T
here are many designs of low-power alarm circuits. Figure 1 shows one of the most commonly used, which is also inexpensive and quick to assemble,
consisting of only a few components – a TTL chip, a Zener diode, resistors and a speaker. Te circuit works as follows: Te TTL
chip is a voltage comparator, which compares the input voltage with the Zener voltage. When the input voltage drops below that of the Zener, the TTL output goes high, turning the speaker on. When the input voltage stays above the Zener voltage, the TTL chip output goes low, which turns off the speaker. A beneficial feature of this circuit is
that it draws very little current in standby mode; i.e., when the input voltage is higher than that of the Zener, namely 144µA. Tis makes it suitable for many applications where power consumption is a concern. For example: • In battery-powered devices the circuit monitors the battery voltage and sounds an alarm if the voltage drops below a certain level.
• In security systems the circuit can monitor a break-in or other alarming event.
• In industrial applications the circuit can monitor the voltage of a power supply or other electrical system. Figure 2 shows this circuit’s performance,
with the x axis representing the input voltage and the y axis the current passing
through the 100-ohm speaker. Treshold voltage is the voltage at which the current through the speaker begins to increase significantly. In our case this is around 5Vdc – the nominal voltage of the Zener diode. When the input voltage is below 5Vdc,
the current passing through the speaker goes high, operating the speaker; above 5Vdc this current is very small. In this case, the maximum current that flows through the speaker is around 144µA. TTL is a type of logic gate that operates
at a supply voltage of 5Vdc. If the supply voltage goes above or below this level, the circuit’s performance will change, too. Standby current is the current that flows through the circuit when it is not actively doing anything. Te standby current of a
Figure 1: A typical peak detector circuit
08 November 2023
www.electronicsworld.co.uk
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