Guide for Limit Switches

Limit Switch Sensors Circuits

The use  of optical sensors to replace mechanical switches is an important feature of today's automation techniques. The mechanical switch is prone to failure after a relatively short operating life. Solid-state sensors have no moving parts to wear, jam, or stick and hence have much longer operating life.

Optical sensors are quiet and require no force to operate. They can be activated by a web of paper or a foil vane attached to a meter movement. No open mechanical contacts are involved. Mechanical contacts wear, pit, and stick. Even low current reed relays are known to stick at the most inopportune time.

Light in Motion offers two families of optical limit switches, a slotted family with tabs H21BX, Figure 1, and a slotted family without tabs, H22BX, Figure 2. Both types employ a photodarlington as the detector and an infrared LED as the light source. The detector sees the infrared light and provides a collector current proportional to the brightness of the light incident on the detector, Figure 3. There are 3 ranges of sensitivity, B1, B2 and B3 in increasing order. In the following developments we will use the H21B1.     
                            Transmissive switch with tabs                                        Transmissive switch without tab                                           
                      Figure 1. H21BX Slotted object Sensor                                   Figure 2. H22BX Slotted object Sensor
                                                with tabs                                                                                    without tabs
                                         Slotted switch schematics
                                                                       Figure 3. H21B1 Circuit Schematics

The electronic circuits used with the optical sensors must be properly designed to match the performance of the photodarlington circuit in the detector. The detector is capable of saturating at IC = 2mA with normal LED drive currents (see data sheets). The load resistance must match this current capability. A typical connection of the H21B1 is shown in Figure 4, and the transfer function of the circuit is shown on the graph of Figure 5. Note that for load resistances above 10K ohm the detector is very sensitive and will function well only with a value of IF less than 5mA or with a completely opaque interrupting vane.

                                                                                 circuit connection
                                                                                Figure 4. Typical H21B1 Circuit Connection

                                                                 transfer function
                                                                  Figure 5. Transfer Function Load Plot for H21B1

Since the H21B1 can easily sink IC = 2mA at VC = 0.7V it can be connected directly to the input of TTL logic as shown in Figure 6.However since the frequency response is quite slow compared to TTL switching requirements, it is recommended that the H21B1 be first connected to a Schmitt trigger, as shown, to improve rise and fall times. A frequency response of 1KHz is adequate for most mechanical motion detection.

                                                                    drive TTL
                                                                     Figure 6. Current Sink for Driving a TTL Schmitt Trigger

A typical 3-wire circuit for using the H21B1 is shown in Figure 7. The value of resistor RD determines IF, and RL determines the load line and sensitivity. 

                                                                                  3 wires circuit
                                                                        Figure 7. 3-Wire Circuit for H21B1

 The circuit of Figure 8 illustrates one method of obtaining 2-wire sensing since the RL and eo terminal can be remote from the sensor. With a Vcc = 20V, RL = RD = 1K and a current transfer ratio or 100%, a signal delta of 2V is obtained at e0 when the LED light is interrupted.

                                                                   2 wires circuit     
                                                                   Figure 8. 2-Wire Circuit for H21B1

A more complicated circuit is shown in Figure 9 where a current booster amplifier is used to shunt the photodarlington sensor. A Zener diode is included to maintain IF when the light is interrupted. Again, 2-wire remote sensing is possible and a signal of ̴ Vz is obtained. Other variations are possible.

                                                                  improved 2 wires circuit
                                                                      Figure 9. Improved 2-Wire H21B1 circuit

The current booster circuit of Figure 9 can be used in a complete all solid-state intrusion alarm circuit as shown in Figure 10. In the intrusion detector loop a series of SCR opto-isolators is installed across each of the sensor segments shown as tapes or switches. So long as the tapes or switches are closed circuits the corresponding LED is not energized by the constant current supply. If any of the tapes or switches are opened, the corresponding SCR is triggered via the LED and sets off an alarm indicator. If the circuit is opened, an excess voltage alarm is triggered, and a current presence alarm also sounds. The H21B1 and its booster keeps the voltage across its LED below threshold so long as the light is not interrupted. If the light is interrupted for only a moment the SCR is triggered. The vane for the H21B1 slot could be always in place with a hole to allow the light through. If the vane is moved, or removed from the slot, the light is interrupted. In this method the H21B1 can replace all mechanical door switches, magnetic window switches, etc. where a small movement will trigger the alarm system.

               Intrusion alarm
                                                                                Figure 10. Intrusion Alarm Detector Circuit


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