Design Your Circuit. Part XI- Schmitt Trigger in Action

Design of the popular Timer IC 555 is so interesting and this tiny chip can be exploited in various ways to make different types of circuits. It can be a Monostable, Astable, Bistable, Toggle, Schmitt trigger etc. The Schmitt trigger action is achieved by exploiting the activities of its two internal comparators. Let us make an Automatic Porch light using the Schmitt trigger property of IC 555. It is an ideal circuit to light the premises of home even if the inmates are not there.

IC 555 as Schmitt trigger

Besides the timing applications, the two comparators of IC 555 can be used independently for other applications. One typical application is the Schmitt trigger design. In this, the inputs of the upper comparator (Pin 6) and the lower comparator (Pin 2) are tied together and biased at ½ Vcc through a potential divider. Since the Threshold comparator (Upper) trips at 2/3 Vcc and the Trigger comparator (Lower) trips at 1/3 Vcc, the bias provided by the potential divider is centered within the comparator’s trip limits. An input signal of sufficient amplitude Sets and Resets the internal Flip-Flop and gives high / low output

When the input voltage of upper comparator decreases below 2/3 Vcc and that of lower comparator below 1/3 Vcc, IC gives a high output from its pin 3.When the upper comparator (pin6) is above 2/3 Vcc, the internal Flip-Flop of IC resets and if the current into the lower comparator (pin2) is more than 1/3 Vcc, output of IC remains low.

Schmitt trigger circuit

An automatic lighting system can be implemented using the IC 555 as a Schmitt trigger. This switchless circuit turns on the lamp in the evening and turns off in the morning. It is ideal to light the premises of the home even if the inmates are not there. Unlike other LDR based circuits, it will not cause lamp flickering during the light transition so that CFL and fluorescent lamps can be used. I have contributed this circuit in http://www.electroschematics .com earlier and reposting the same here also.

LDR and timer IC 555 are used in the circuit for automatic switching. Light Dependent Resistor offers very high resistance around 10 Meg ohms in dark but in light it has only 100 K or less resistance. So it is an ideal component to switch on lamps based on the presence or absence of sun light.

In the circuit, the inputs (pin6 and pin2) of both the comparators are shorted and connected to the junction of LDR and the Preset VR1. In day light, LDR passes more current and the current into the upper comparator (pin6) is above 2/3 Vcc. This resets the internal Flip-Flop of IC. At the same time, the current into the lower comparator (pin2) is more than 1/3 Vcc. Both these condition causes low output from IC1.

When the light falling on the LDR decreases, its resistance increases, and the current flowing to the upper and lower comparators of IC1 decreases. The input voltage of upper comparator decreases below 2/3 Vcc and that of lower comparator below 1/3 Vcc. This causes, high output from pin 3 of IC1. This triggers T1 and the relay connected to its collector turns on. The lamp gets AC through the Common and NO (Normally Open) contacts of the relay. Only the Phase line is connected through the relay contacts. The neutral line goes as such.

VR1 adjusts the sensitivity of LDR at the particular light level at which the lamp turns on. Capacitor C2 maintains the base voltage of T1 for clean switching action and also avoids relay clicking. Diode D1 removes back e.m.f when T1 switches off.
Note: LDR should be placed away from the Lamp light but it should get day light. Use 6 Volt PCB relay.

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