Annunciator circuits produce audible signals when faults or unusual activities occur in a system or a process. They are crucial for safety purposes, as they alert operators to avoid accidents or improper procedures. Annunciator circuits are found in security, burglary, fire, machine malfunction and medical systems.
The circuit shown in Figure 1 represents a typical annunciator circuit, widely used due to its simplicity and ease of construction. It is designed to activate an audible alarm through a speaker or buzzer, shown as R5 in the figure. Figure 2 shows the waveform generated by the circuit.
It is important to ensure the reliability and functionality of these circuits under all possible circumstances, including when power supply voltage begins to decay.
This experiment should show us the limitations of the annunciator circuit when the power supply voltage drops below the standard 5V.
Figure 1: A popular annunciator circuit
Figure 2: Current passing through the speaker or buzzer
The experiment
We used the circuit shown in Figure 1 for the experiment, and applied a range of voltages to it, with the peak-to-peak output current passing through the speaker, R5. All the monitored current peak-to-peak values were plotted against each applied voltage, which was varied from 0Vdc to 15Vdc in 1Vdc steps; see Figure 3. All electronic components were kept fixed throughout this experiment.
Figure 3 shows that the circuit is very stable at the higher end of the power supply range, specifically when it exceeds 5Vdc, up to 15Vdc. However, when the voltage decreases, the situation changes: Below 7Vdc, the circuit’s output starts to decline significantly. At 4Vdc, the circuit stops functioning, with no output observed.
Figure 3: Peak-to-peak output current passing through the speaker
This finding is important when building circuits, to ensure the power supply is higher than 5Vdc, otherwise this circuit won’t be reliable and its proper functioning will be compromised.
By Dr Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions
