Jean-Bernard Guiot
EDN
Figure 1a shows a latching circuit based on an optocoupler, IC1. If the off switch remains closed, pressing the on switch powers the LED in the optocoupler. Thus, the transistor in IC1 turns on. You can now release the on switch, and the transistor remains on. You must adapt the current-limiting resistor, R, to the power-supply voltage and the optocoupler's characteristics. To turn off this latch, press the off switch, thereby cutting the current path. The output of this circuit comes directly from the collector of the transistor. If you need an isolated output, you can use the circuit in Figure 1b, which works similarly to the circuit in Figure 1a. Figure 1c shows a similar circuit but with an NO (normally open) switch for both the off and on switches. Shorting the LED in IC1 turns off the transistor in IC1. Thus, IC2 also turns off. Note that simultaneously pressing both the off and the on buttons turns on the output. But be aware that pressing the off button reduces the voltage across the circuit below the resistor by 1 to 2 V; you must consider this fact when you calculate the value of R.
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| Figure 1. | Various optocoupler-latching circuits are suitable for motor control. In (a) and the isolated circuit (b), simply pushing the on button turns on the circuit. In (c), you push both buttons simultaneously. |
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Figure 2 is a simplified schematic of an application in which a motor must always stop at a defined position. "Simplified" means that it shows only the parts needed for illustrating the optocoupler's role and omits protection, correct polarization, and a braking circuit, for example. IC1 is the latching optocoupler. IC2 is an npn-transistor, NO proximity switch. The proximity switch is on when metal is near its sensing area. A cam with a notch mounts on the motor axis. The motor should stop when the notch passes the sensing area of the proximity switch. Closing the on switch turns on transistor QOUT, thus allowing the motor to rotate, regardless of the status of the rest of the circuit. When you open the on switch, two things can happen:
- The notch is near the sensing area of the proximity switch. IC1 and IC2 are both off, so QOUT receives no base current. Therefore, the motor does not rotate.
- Metal is near the sensing area of the proximity switch, turning on the switch. The LED and, therefore, the transistor, in IC1 are still both on. Thus, the motor continues to rotate until the notch nears the sensing area of the proximity switch. At this point, the motor ceases to rotate.
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| Figure 2. | A notched cam controls a proximity switch, which determines the motor’s stop positions. |
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All circuits were tested using a PC814 optocoupler from Sharp. (The circuit in Figure 2 has been running for a few months on several machines.) Optocouplers, such as the 4N33, with accessible transistor bases are more difficult to use in this application. Of course, these circuits cannot replace all relays. But they are convenient and effective in applications in which the current and voltage are in the range of optocouplers. (You can add some amplification by adding some power Darlington transistors.) The main advantages of these types of circuits are that they are low-power, have no mechanical noise, have no switch bounce, cost only about 50 cents, require no critical components, and measure less than 6×10 mm.
