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 (VPOUT
) from the high-side power monitor, U1:
= 40.96 × VSENSE
is the voltage between RS+ and RS-, and AVPOUT
is the voltage gain from VSENSE
high-side power and current monitor is chosen for its AVPOUT
value of 40.96. VSENSE
equals 25mV/V x VCC
in this case. As shown in Figure 1, you can calibrate VIN
to adjust the level of POUT
as required by the application.
The full-scale value for a typical sensor (the maximum value of VSENSE
) might equal 25mV/V × VCC
. For now, we assume that 20mV/V (less than full scale) represents a hazardous condition. Therefore, at the alarm condition:
= 40.96 × 20mV/V × VCC
= 0.8192V/V × VCC
can be adjusted through the VIN
input, you can use fixed thresholds for the alarm and warning conditions:
= [(R4 + R5)/(R3 + R4 + R5)]VCC
= 0.6 × VCC
= [R5/(R3 + R4 + R5)]VCC
= 0.5 × VCC
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 VALARM
condition, and obtain VWARNING
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 VPOUT
. The maximum voltage at VIN
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 VCC
in the range 4.4V to 4.5V. Because the circuit operation is ratiometric, variations in VCC
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 VBATT