Nuts & Volts - April 2005

Universal Relay Board

an output swing of almost full rail to rail; here it is 12 volts positive to zero volts negative. Only half of the LM358 is used here for all configurations.

The op-amp is set up as a non-inverting comparator in any of the configurations. This means that it will toggle the output of the LM358 high whenever the inverting, negative input, Pin 2, is more negative than the voltage at its positive input, Pin 3. Otherwise, it will toggle the output low when vice-versa input voltages are applied. Via VR1, the negative input can be set to sense any voltage from rail to rail and positive to negative (except in the sound-activated configuration which is highly biased toward the negative rail). The transition between high and low output on the LM358 only needs one of the inputs to be around five millivolts higher or lower, which is very sensitive. The output of the microphone is around 100 millivolts at its maximum output.

The output of the LM358 passes through D2 IN4148, then charges up C1 (a 1 µF electrolytic capacitor), which acts as a one second sample and hold. It can’t discharge back through D2, but only through R3 by increasing the value of C1 to about 47 µF. As a result, you will get a very long latching effect, close to one minute in length.

Originally, I thought a junction field-effect transistor (JFET) might be able to drive the relay, but instead I found a much better option, the 2N7000. Therefore, R1 was left onboard. The reason for this is that field-effect transistors (FET) draw very little current to switch fully; they rely mainly on the voltage on their gates to work. Usually, JFETs need a negative voltage on their gates to turn off fully (below the actual zero or negative rail), but the 2N7000 metal-oxide semiconductor field-effect transistors (MOSFET) are much easier to use. Most JFETs don’t have enough drive capability for a relay anyway.

The 2N7000 does not need the negative gate bias. It is actually an N-channel-enhancement-mode MOSFET. A bi-polar transistor is unsuitable in this circuit (mainly for the microphone version), as it needs too much base input current to operate correctly, but a FET that only needs voltage works fine.

This particular FET can handle up to + 30 volts on its gate input, but it can’t have any current driving it. More than a few milliamps will kill these devices very quickly.

The R1 serves several purposes. To save power, it reduces the current drawn by the relay, which may be an advantage if a battery supply is used. Also, it

APRIL 2005

Figure 2. The mic’s FET needs to be powered via R5, a 22 kilohm resistor.

can be used to run the circuit a few volts above the relay voltage if needed. R1 could be replaced with a wire link. The voltage drop across R1 does not affect the relay (it could be replaced with a wire link), while a 200-ohm resistor would be enough to stop the relay from fully energizing.

Other TO- 92 Package MOSFET Devices that may work are the following:

• Vishay, TP0610L/VP0610L/BS250, I/DS around 180 mA.

• VN0300L, I/DS around 200 mA.

• Fairchild BS170, I/DS around 500 mA.

Check for correct pinouts before using other devices. These devices can be damaged by static electricity; take care.

If an LM358 op-amp is not available, you could try one of the many dual eight-pin op-amps, as long as it can run on a single supply voltage and has a reasonable O/P voltage swing. Just check the spec sheet on any other device you decide to use, as many of these dual op-amps are pin-for-pin compatible.

Table 1. This table is for R5 and R6 at 12 volts DC.

High Side + Low Side - Voltage = the value at the resistor’s junction to the zero-to-negative rail.

R5 = 10KΩ R5 = 15KΩ R5 = 22KΩ R5 = 33KΩ R5 = 39KΩ R5 = 47KΩ R5 = 56KΩ R5 = 68KΩ R5 = 82KΩ R5 = 100KΩ

R6 = 100KΩ R6 = 82KΩ R6 = 68KΩ R6 = 56KΩ R6 = 47KΩ R6 = 47KΩ R6 = 39KΩ R6 = 33KΩ R6 = 22KΩ R6 = 10KΩ

Voltage = 10. 9 V - 11.0 V & above = ON, 10. 8 V & below = OFF Voltage = 10.1 V - 10.2 V & above = ON, 10.0 V & below = OFF Voltage = 9.0 V - 9.1 V & above = ON, 8. 9 V & below = OFF Voltage = 7. 5 V - 7. 6 V & above = ON, 7. 4 V & below = OFF Voltage = 6. 5 V - 6. 6 V & above = ON, 6. 4 V & below = OFF Voltage = 6.0 V - 6.1 V & above = ON, 5. 9 V & below = OFF Voltage = 4. 9 V - 5.0 V & above = ON, 4. 8 V & below = OFF Voltage = 3. 9 V - 4.0 V & above = ON, 3. 8 V & below = OFF Voltage = 2.5 V - 2.6 V & above = ON, 2.4 V & below = OFF Voltage = 1.09 V - 1.19 V & above = ON, 0.99 V & below = OFF