A Single-Chip Personal Alarm

Abstract

This application note presents a single-chip alarm circuit that issues a warning, then an alarm, when toxic or explosive gases are sensed. The circuit is simple and low cost, while offering moderate precision.

This design idea was published in the online edition of Portable Design magazine in April 2006.

Workers in the mining industry and other industrial environments can be subject to toxic or explosive gases. They can benefit from inexpensive personal monitors. While some applications require detailed data logging and information gathering, others require merely a warning signal or alarm. This single-chip alarm circuit offers a modest level of precision and is simple and inexpensive.

Gas sensors often use a bridge configuration. Typically, the bridge output goes to an instrumentation amplifier, and then to an A/D converter or adjustable reference for use as an alarm or display. The circuit of Figure 1 operates from a 9V battery and monitors the output of a sensor bridge. It provides a visual warning and alarm in response to a hazard. The monitored signal appears as a voltage output (V_POUT) from the high-side power monitor, U1:

V_POUT = A_VPOUT × V_SENSE × V_IN = 40.96 × V_SENSE × V_IN

Where V_SENSE is the voltage between RS+ and RS-, and A_VPOUT is the voltage gain from V_SENSE to P_OUT.

The MAX4211F high-side power and current monitor is chosen for its A_VPOUT value of 40.96. V_SENSE equals 25mV/V x V_CC in this case. As shown in Figure 1, you can calibrate V_IN to adjust the level of P_OUT as required by the application.

The full-scale value for a typical sensor (the maximum value of V_SENSE) might equal 25mV/V × V_CC. For now, we assume that 20mV/V (less than full scale) represents a hazardous condition. Therefore, at the alarm condition:

V_POUT = 40.96 × 20mV/V × V_CC × V_IN = 0.8192V/V × V_CC × V_IN

Because P_OUT can be adjusted through the V_IN input, you can use fixed thresholds for the alarm and warning conditions:

V_ALARM = [(R4 + R5)/(R3 + R4 + R5)]V_CC = 0.6 × V_CC

V_WARNING = [R5/(R3 + R4 + R5)]V_CC = 0.5 × V_CC

Figure 1. This single-chip circuit monitors a sensor signal represented by the bridge voltage. It produces first a warning, and then an alarm as the hazard signal intensifies.

This dual-threshold, single-adjustment design lets you calibrate the V_ALARM condition, and obtain V_WARNING as an uncalibrated early-warning flag. To calibrate, carefully apply monitored gas to the sensor in the concentration necessary for alarm. Then, adjust potentiometer R2, as required, to set V_POUT. The maximum voltage at V_IN should be 1.0V, and the potentiometer should provide at least 10 turns of adjustment.

A simple NPN shunt using a 3904 transistor and 5.1V zener diode generates V_CC in the range 4.4V to 4.5V. Because the circuit operation is ratiometric, variations in V_CC do not affect the output accuracy. The value of base resistor R1 (3.01kΩ) ensures sufficient base current to saturate the transistor when battery voltage is low (6.0V).

Red and yellow LEDs provide the alarm and warning lights. If the warning light is not required, you can set up a low-battery detector using the liberated comparator, the internal 1.21V reference, and an external voltage-divider connected to V_BATT.