How does a bistable switch made with a comparator work?
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How does a bistable switch made with a comparator work?

Posted Date: 2024-02-01

Earlier in my article I proposed a new class of bistable components - dual threshold thyristors, when two non-zero level ("high" or "low") control voltages are applied to the thyristor input , the thyristor switches from one state to another.

A bistable load switch (Figure 1) is designed to switch a load when Uon or Uoff voltage is applied to the input of the device. The device contains two comparators U1.1 and U1.2, and an output transistor Q1, such as a 2N7000.

Figure 1 A bistable switch controlled by the input voltage level with individually adjustable load turn-on and turn-off thresholds.

The device works as follows. Its input terminal (inverting input of comparators U1.1 and U1.2) briefly provides a certain level of voltage (Uon or Uoff). The non-inverting input of the comparator comparison is the two-level voltage provided by potentiometers R2 and R3. Comparator U1.1 switches when the switching voltage Uon (Uon

In order to return the device to its initial state (disconnecting the load), a higher level voltage (Uoff) needs to be applied to the input, which switches the state of the second comparator U1.2. When this comparator is switched, the voltage at the inverting input of comparator U1.1 drops to zero and the circuit returns to its initial state.

Such a device, with some simplifications and modifications, can be placed in a DIP6 base, Figure 2. When the low-level voltage Uon is briefly applied to the input terminal of the device, the output signal level switches from the condition level 0 to 1, and when the high-level voltage Uoff is applied to the input terminal, it returns to the initial state.

Figure 3 shows a typical circuit for turning on such a chip. External adjustment elements R1 and R2 are used to adjust the switching thresholds for on and off (Uthr1 and Uthr2).

Figure 2 A bistable switch, and possible integrated circuits based on it.

Figure 3 Variants of a bistable switching chip with an external switching threshold control circuit or an internal unregulated circuit, and the possibility of using this circuit in a DIP4 base to obtain an unregulated version with a fixed switching threshold.

If resistor dividers R1–R3 are used to set the constant on and off levels of Uthr2 and Uthr1, the bistable switch can be placed in a DIP4 chip base as shown in Figure 3, which has the power terminals and Input and output terminals. To obtain switching levels that are independent of the supply voltage, a simple voltage regulator (Zener diode) built into the microcircuit can be used to power the resistor divider R1–R3.


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