In this post we learn how to build 20 simple IC 555 based application circuits which for many useful purposes.
This post begins by looking at the 555 as the main element in a Schmitt trigger circuit. It proceeds to go into detail the function of the 555 in several astable multivibrator or oscillator circuits several useful applications. Those circuits consist of light and dark along with hot and cold activated alarms. Additional circuits are a code practice oscillator, a door buzzer, a continuity tester, a signal generator, and a metronome. Various light actuator and relay driver circuits are also further enclosed.
IC 555 Block Diagram
Figure 1 is the pinout and functional block diagram for the 555 timer IC. From our earlier discussions we know that for a 555 in the delay timer mode, the delay could be accurately managed through a single external resistor and one capacitor.
When operated as an astable oscillator, the frequency and duty cycle could be precisely governed with a couple of external resistors and a solitary capacitor.
You may want to remember that the 555 could be activated and reset with falling waveforms, and the output circuit can easily source or sink up to 200 milliamperes, or operate TTL circuits. The 555's characteristics include normally on and normally off outputs.
IC 555 Schmitt Trigger
Figure 2-a shows the 555 IC as the active element in a Schmitt trigger circuit. Observe that the 555's TRIGGER pin 2 and THRESHOLD pin 6 are linked to create an input terminal. External input signals are utilized straight at that position. The OUTPUT pin 3 works like the output terminal.
Internal comparators A and B (see Fig. 1) are biased through an in built voltage divider stage.
That divider biases comparator A with 2/3rd of the supply voltage, and the non-inverting pin of comparator B with 1/3rd of the supply voltage. Comparator A runs the R input and comparator B runs the S input of the in-built R-S flip-flop.
In the event the input circuit voltage in Fig. 2-a increases over 2/3rd of the supply voltage, the 555 output turns to 0V. It continues to be in this position until the input voltage drops under 1/3rd of the supply voltage.
This causes the output pin#3 to turn high and continue to be high until the input goes up over the 2/3rd supply level again. The difference between these a couple of trigger ranges is known as the hysteresis value.
It is actually 1/3rd of the supply in Fig. 2-a. which is a significant hysteresis causing the circuit to be beneficial in signal processing where noise and ripple needs to be terminated, as demonstrated in Fig. 2-b.
IC 555 Circuit with Hysteresis
Figure 3 exhibits the way the circuit in Fig. 2-a could be improved into a high-performance sine-to-square-wave converter helpful at input frequencies up to approximately 150 kHz.
The voltage divider established by R1 and R2 biases the input pinout (pins 2 and 6) of the 555 at its quiescent value of 50% of the supply voltage (i.e., at the center level between the lower and upper trigger magnitudes).
The sine-wave input signal is superimposed on this position with capacitor C1. Square-wave output signals are extracted from pin 3 of the IC. Resistor R3 is rigged in series with the input pin to make sure that the sine-wave signal will not be deformed once the 555 is turned ON.
Figure 4 demonstrates the way the Schmitt trigger circuit could be converted to a dark-activated relay trigger switch by connecting the light dependent voltage divider made up of potentiomenter R1 and photocell R2 to the input pin of the IC. The potentiometer and photocell resistance magnitudes are almost identical on the center of the light-activation range.
The in-built feature of a high input hysteresis of the Schmitt trigger restricts the effectiveness of this circuit to quite specific light-sensing circuit applications.
Darkness Activated Switch with Relay
A much more valuable relay based, darkness triggered switch circuit is demonstrated in Fig. 5. It works like a quick comparator instead of a genuine Schmitt trigger.
The THRESHOLD pin 6 to internal comparator A of the 555 is attached entirely high through resistor R3, while the output of the light sensing potentiometer R1 and photocell R2 voltage divider is utilized to TRIGGER pin 2 of comparator B.
The photoresistive ingredient for this circuit could be almost any cadmium-sulfide photocell carrying a resistance between 470 ohms and 10 kilohms at the desired turn-on light level. The circuit in Fig. 5 also can work as a light (instead of dark) triggered switch by swapping the placements of the potentiometer and photocell, as shown in Fig. 6-a.
The circuit could also work as a temperature stimulated switch simply by replacing a thermistor having a negative temperature coefficient for the photocell, as indicated in Figs. 6-b and 6-c. (A thermistor having a negative temperature coefficient lowers in resistance with increase in its temperature).
The thermistor for this program need to have a resistance value in between 470 ohms and 10 kilohms for the intended temperature switch ON. Thermistors are usually manufactured as radially terminated disks, and their resistance values are set at 25° C.
Code Practice Oscillator Circuit
Figure 7 indicates the 555 as the semiconductor IC in a Morsecode practice oscillator. The circuit is an oscillator having a adjustable frequency range from 300 Hz to 3 kHz by altering tone control potentiometer R3.
The volume of the headphone Z1 could be adjusted through potentiometer R4, and the headphones can exhibit any DC resistance from a low value ohms to many megohms.
The oscillator circuit consumes zero quiescent current as long as the normally open Morse key remains unconnected with the 5 to 15 volt supply.
Door Buzzer Circuit
Figure 8 demonstrates the 555 functioning like a semiconductor unit in a easy electronically triggered door buzzer. Push button switch S1 links the 555 to the 9 -volt battery, and the output of the IC is connected to speaker SPKR1 by means of capacitor C4. Capacitor Cl constitutes a low supply line impedance, ensuring sufficient output drive current to the speaker as soon as S1 is actuated. The circuit produces a continuous buzzing sound as established by potentiometer R2.
Continuity Tester Circuit
Figure 9 exhibits the 555 as the semiconductor element within a continuity tester which produces an audible note only when the resistance between the test probes is lower than a some ohms.
The circuit's functioning is determined by an output sound that becomes audible only when the RESET (pin 4) is biased positive to around 600 millivolts or higher using the sensitivity potentiometer R5. Pin 4 is usually drawn to ground through resistor R2, thus no sound frequency is audible.
For the buzzer in the circuit of Fig. 9 to activate, both probe points need to come in contact, linking R2 to the output of the reference generator created by resistor R3 and Zener diode D1 by means of sensitivity potentiometer R5.
Potentiometer R5 has to be meticulously tweaked to ensure that a buzzer sound is hardly hearable. As a result, when the resistance across the probe ends is greater than a couple of ohms while implementing a continuity test, the buzzer audio isn't heard.
The circuit works using a few milliamperes anytime S1 is activated, even if the probe ends aren't in contact.
Signal Generator Circuit
Figure 10 displays the 555 working like a signal generator for assessing both audio and radio frequency circuits. The circuit works with a frequency of a several 100 hertz when S1 is actuated.
Its square -wave output is extremely abundant with harmonics, and these could be noticed at frequencies as much as tens of megahertz using a radio receiver. The signal level could be adjusted by tweaking the potentiometer R3.
When the output goes high, C1 charges swiftly by means of diode D1 and resistor R1 in series to create a frequency pulse of just a few milliseconds long.
If the output turns low again, C1 discharges by means of potentiometer R3 and resistor R2 in series to deliver an off period of around 2 seconds (30 pulses each minute).
The output pulses are given to speaker SPKR 1 via level control potentiometer R4 and buffer transistor Q1.
LED flashers and alarms.
Figures 12 to 14 demonstrate 555 in LED flasher circuits where the LED's work with identical on / off operating periods. Using the part values given n the diagrams, each circuit blinks at about one flash per second cycle. The circuit in Fig 12 carries a single ended output.
Sometimes just one LED and sometimes LED's in series could be used between the OUTPUT pin3 and GROUND pin l of the 555, with all LED's turning on/OFF together. Resistor R3 fixes the operating current of the LED's.
The circuit in Fig. 13 resembles the circuit in Fig. 12, although it features a double-ended output relationship. The LED's on top of pin 3 are on if the LED's under pin 3 are shut off, and vice versa. Resistor R3 determines the switch ON currents of the lower LED's, and resistor R4 fixes the working currents of the upper LED's.
Figure 14 exhibits how you can customize the circuit in Fig. 12 for programmed darkness switching. Resistors R3 and R4, photocell R1, and potentiometer R2 constitutes a light detected Wheatstone bridge that activates the 555 through bridge balance detector Q1 and the RESET pin 4 of the IC. The oscillator is usually deactivated through resistor R6, that drags RESET pin 4 near 0 V.
Figure 15 illustrates an automated heat or light activated relay driver. The circuit operates using any 12 -volt relay with a coil resistance higher than about 60 ohms. As soon as powered, the circuit activates the relay RY1 on / off about 1 Hz. A heat or light activated non-stop alarm circuit is demonstrated in Fig. 16.
As soon as activated, this circuit produces a whizzing noise at approximately 800 Hz. Many watts of power tend to be consumed by the speaker SPKR1 via buffer transistor Q2.
The resulting large speaker output current could possibly send ripple voltage towards the power supply hence diode D1 and capacitor C3 are included to safeguard the circuit from this interference. Diodes D2 and D3 eliminate the inductive switching surges of the speaker, protecting Q2 from a possible destruction.
Other kinds of sensor circuits which could automatically trigger the circuits of both Figs. 15 or 16 are presented in Fig. 17. In case light depended operation is expected, the sensor can be a cadmium sulfide photocell. In case the circuit is required to be activated in response preset value set for a dark actuation, the circuit given in Fig 17-a could be applied. If the circuit is to be activated once the light strength increases to some preset value, the circuit of Fig 17-b has to be employed.
If you would like temperature based activation, work with a thermistor having a negative temperature coefficient as the sensor. For low temperature functioning, you may try the circuit of Fig. 17-c; for higher temperature functioning, try the circuit of Fig. 17-d. Irrespective of the type of functioning preferred, the sensor component will need to have a resistance value between 470 ohms and 10 kilohms on the preferred trigger level.
A 555 can easily work like a outstanding button activated relay driving timer if it is hooked up as an monostable mode. In real applications, this kind of a circuit is not going to create precise timing signals greater than several minutes simply because they need an electrolytic capacitor having a large capacitance value. Electrolytic capacitors normally include large tolerance values (- 50 to +100%) and huge and capricious leakage currents. When the 555 is required to be the effective element in long interval timers, the associated circuitry should incorporate a capacitor apart from an electrolytic.
60 Minute Timer
Figure 18 displays, as a block diagram, the guidelines that's used for a design for a 60 -minute relay based timer. In cases like this, the 555 is arranged in the astable mode. It has its output attached to the relay driver via a 14 -stage binary divider IC.
That wiring set up allows a standard division ratio of 16,384. In case the output of the 555 is fixed to zero in the beginning of an input count, the output may turn high upon receiving the 8192nd input pulse. The circuit will continue to be high until the 16,382nd pulse is reached.
Then, the output may turn low once again, finishing the standard working routine. In Fig. 18, the timing pattern is started by activating S1, that attaches the supply with the circuit, at the same timeinitiating the oscillator and establishing the counting to zero by means of capacitor C2 and resistor R3. This turns the counter output to 0V switching ON the relay.
The contacts of RY1 hold the power supply connection the moment S1 is released. This situation is preserved till the 8192nd oscillator pulse gets to the input pin of the counter.
When this happens the counter output turns to the positive supply level switching OFF the relay. This causes the relay contacts to open, removing the supply from the circuit and finishing the working cycle.
In this circuit, the oscillator has to run through a sequencing interval that may be 1/8192nd of the required timing period (0.44 second in our case). This may be accomplished using a 1 uF polyester capacitor and a resistor of approximately 300 k.
Figure 19 exhibits the way the design in Fig. 18 is executed to create an effective relay based timer circuit ideal for 1 to 100 minutes in 2 consecutive decade ranges. This circuit is driven through a 12 -volt supply. The relay should have a coil resistance of 120 ohms or higher.
IC 555 Long Duration Timer Circuit
Figure 20 demonstrates what sort of time delay of the circuit in Fig. 19 could be prolonged by hooking up an extra divider step between the output pin 3 of the 555 and the input of the relay driving output. In this particular circuit a divide-by-ten 4017B CMOS IC is attached between the output of the 555 and the 4020B 14 -stage binary counter.
The set up in Fig. 20 offers an appropriate all round division ratio of 81,920, which enables a delay from 100 minutes to 20 hours obtainable through this single range timer. Note that each of the divider IC's are reset by the series blend of capacitor C3 and resistor R3 automatically as soon as switch S1 is switched ON.
Figure 21 exhibits the method to customize the circuit in Fig. 20 to generate a wide range standard timer which ranges 60 seconds or so to 20 hours in 3 decade based ranges. The divide-by-ten stage becomes operational as long as switch S1 -a is moved to position 3.