Timer circuits using 555 ic

Example Circuits:

Tony has placed a couple 555 circuit examples below for your convenience. Play with different component values and use the formulas . Things to remember: For proper monostable operation with the 555 timer, the negative-going trigger pulse width should be kept short compared to the desired output pulse width. Values for the external timing resistor and capacitor can either be determined from formulas. However, you should stay within the ranges of resistances shown earlier to avoid the use of large value electrolytic capacitors, since they tend to be leaky. Otherwise, tantalum or mylar types should be used. (For noise immunity on most timer circuits I recommend a 0.01uF (10nF) ceramic capacitor between pin 5 and ground.) In all circuit diagrams below I used the LM555CN timer IC from National, but the NE555 and others should not give you any problems.

Dark Detector     Power Alarm

Tilt Switch     Elec-Eye Alarm

Metronome     CW Keyer

Contineous Wave (CW) Monitor     10-Min. Timer

Schmitt Trigger     Better Timing

Missing Pulse

Circuits 1 to 10a:
Play with different indicating devices such as bells, horns, lights, relays, or whatever (if possible). Try different types of LDR's. If for any reason you get false triggering, connect a ceramic 0.01uF (=10nF) capacitor between pin 5 (555) and ground. Keeping the basic rules of the 555 timer, try different values for Ct and Rt (or the C & R over pins 2, 6 & 7) Replace Rt with a 1 megohm potentiometer if you wish. Make notes of the values used and use the formulas to calculate timing. Verify your calculations with your timing.

Fig. 1, Dark Detector:
It will sound an alarm if it gets too dark all over sudden. For example, this circuit could be used to notify when a lamp (or bulb) burns out. The detector used is a regular cadmium-sulphide Light Dependent Resistor or LDR, for short, to sense the absence of light and to operate a small speaker. The LDR enables the alarm when light falls below a certain level.

Fig. 2, Power Alarm: This circuit can be used as a audible 'Power-out Alarm'. It uses the 555 timer as an oscillator biased off by the presence of line-based DC voltage. When the line voltage fails, the bias is removed, and the tone will be heard in the speaker. R1 and C1 provide the DC bias that charges capacitor Ct to over 2/3 voltage, thereby holding the timer output low (as you learned previously). Diode D1 provides DC bias to the timer-supply pin and, optionally, charges a rechargeable 9-volt battery across D2. And when the line power fails, DC is furnished to the timer through D2.

Fig. 3 Tilt Switch: Actually really a alarm circuit, it shows how to use a 555 timer and a small glass-encapsulated mercury switch to indicate 'tilt'.
The switch is mounted in its normal 'open' position, which allows the timer output to stay low, as established by C1 on startup. When S1 is disturbed, causing its contacts to be bridged by the mercury blob, the 555 latch is set to a high output level where it will stay even if the switch is returned to its starting position. The high output can be used to enable an alarm of the visual or the audible type. Switch S2 will silent the alarm and reset the latch. C1 is a ceramic 0.1uF (=100 nano-Farad) capacitor.

Fig. 4, Electric Eye Alarm: The Electric-Eye Alarm is actually a similar circuit like the Dark Detector of Fig. 1. The same type of LDR is used. The pitch for the speaker can be set with the 500 kilo-ohm potentiometer. Watch for the orientation of the positive (+) of the 10uF capacitor. The '+' goes to pin 3.

Fig. 5, Metronome: A Metronome is a device used in the music industry. It indicates the rhythm by a 'toc-toc' sound which speed can be adjusted with the 250K potentiometer. Very handy if you learning to play music and need to keep the correct rhythm up.
Error fixed with thanks to Grant Fair in regards to the two resistors. (Grant also added a PNP power transistor to increase the volume and a led for visual as well as audio output).

Fig. 6, CW Practice Oscillator: CW stands for 'Continuous Wave' or Morse-Code. You can practice the morse-code with this circuit. The 100K potmeter is for the 'pitch' and the 10K for the speaker volume. The "Key" is a morse code key.

Fig. 7, CW Monitor: This circuit monitors the morse code 'on-air' via the tuning circuit hookup to pin 4 and the short wire antenna. The 100K potmeter controls the tone-pitch.

Fig. 8, Ten-Minute Timer: Can be used as a time-out warning for Ham Radio. The Federal Communications Commission (FCC) requires the ham radio operator to identify his station by giving his call-sign at least every 10 minutes. This can be a problem, especially during lengthy conversations when it is difficult to keep track of time. The 555 is used as a one-shot so that a visual warning indicator becomes active after 10-minutes. To begin the cycle, the reset switch is pressed which causes the 'Green' led to light up. After 10 minutes, set by the 500K potentiometer R1, the 'Red' led will light to warn the operator that he must identify.

Fig. 9, Schmitt Trigger: A very simple, but effective circuit. It cleans up any noisy input signal in a nice, clean and square output signal. In radio control (R/C) it will clean up noisy servo signals caused by rf interference by long servo leads. As long as R1 equals R2, the 555 will automatically be biased for any supply voltage in the 5 to 16 volt range. (Advanced Electronics: It should be noted that there is a 180-degree phase shift.) This circuit also lends itself to condition 60-Hz sine-wave reference signal taken from a 6.3 volt AC transformer before driving a series of binary or divide-by-N counters. The major advantage is that, unlike a conventional multivibrator type of squares which divides the input frequency by 2, this method simply squares the 60-Hz sine wave reference signal without division.

Fig. 10, Better Timing: Better and more stable timing output is created with the addition of a transistor and a diode to the R-C timing network. The frequency can be varied over a wide range while maintaining a constant 50% duty-cycle. When the output is high, the transistor is biased into saturation by R2 so that the charging current passes through the transistor and R1 to C. When the output goes low, the discharge transistor (pin 7) cuts off the transistor and discharges the capacitor through R1 and the diode. The high & low periods are equal. The value of the capacitor (C) and the resistor (R1 or potmeter) is not given. It is a mere example of how to do it and the values are pending on the type of application, so choose your own values. The diode can be any small signal diode like the NTE519, 1N4148, 1N914 or 1N3063, but a high conductance Germanium or Schottky type for the diode will minimize the diode voltage drops in the transistor and diode. However, the transistor should have a high beta so that R2 can be large and still cause the transistor to saturate. The transistor can be a TUN (europe), NTE123, 2N3569 and most others.

Fig. 10a, Missing Pulse Detector (Basic): This transistor can be replaced with a ECG or NTE159. This is just a basic model but works. Experiment with the values of Resistor and Capacitor. A good example would be the 'Crashed Aircraft Locator' beacon used in radio control. If there is no signal it sees it as a missing pulse and sounds buzzer.

The following circuits are examples of how a 555 timer IC assist in combination with another Integrated Circuit. Again, don't be afraid to experiment. Unless you circumvent the min and max parameters of the 555, it is very hard to destroy. Just have fun and learn something doing it.

555 Two-tone experiment

555 Recorder Beep
  Coin Toss

555 Logic Probe 
with Pulse

Circuits 11 to 14:
Play with different indicating devices such as bells, horns, lights, relays, or whatever (if possible). Try different types of LDR's. If for any reason you get false triggering, connect a ceramic 0.01uF (=10nF) capacitor between pin 5 (555) and ground. In all circuit diagrams below I used the LM555CN timer IC from National. The 555 timer will work with any voltage between 3.5 and 15volt. A 9-volt battery is usually a general choice. Keeping notes is an important aspect of the learning process.

Fig. 11, Two-Tones: The purpose of this experiment is to wire two 555 timers together to create a 2-note tone. If you wish, you can use the dual 556 timer ic.

Fig. 12, Recording Beep: This circuit is used to keep recording of telephone conversations legal. As you may know, doing otherwise without consent of the other party is illegal. The output of IC1 is fed to the 2nd 555's pin 3 and made audible via C2 and the speaker. Any 8-ohm speaker will do.

Fig. 13, Coin Toss: Electronic 'Heads-or-tails' coin toss circuit. Basically a Yes or No decision maker when you can't make up your mind yourself. The 555 is wired as a Astable Oscillator, driving in turn, via pin 3, the 7473 flip-flop. When you press S1 it randomly selects the 'Heads' or 'Tails' led. The leds flashrate is about 2Khz (kilo-Hertz), which is much faster than your eyes can follow, so initially it appears that both leds are 'ON'. As soon as the switch is released only one led will be lit.

Fig. 14, Logic Probe: Provides you with three visible indicators; "Logic 1" (+, red led), "Logic 0" (-, green led), and "Pulse" (yellow led). Good for TTL and CMOS. The yellow or 'pulse' led comes on for approximately 200 mSec to indicate a pulse without regards to its width. This feature enables one to observe a short-duration pulse that would otherwise not be seen on the logic 1 and 0 led's. A small switch (subminiature slide or momentary push) across the 20K resistor can be used to keep this "pulse" led on permanently after a pulse occurs.
In operation, for a logic 0 input signal, both the '0' led and the pulse led will come 'ON', but the 'pulse' led will go off after 200 mSec. The logic levels are detected via resistor R1 (1K), then amplified by T1 (NPN, Si-AF Preamplifier/Driver), and selected by the 7400 IC for what they are. Diode D1 is a small signal diode to protect the 7400 and the leds from excessive inverse voltages during capacitor discharge.
For a logic '1' input, only the logic '1' led (red) will be 'ON'. With the switch closed, the circuit will indicate whether a negative-going or positive-going pulse has occurred. If the pulse is positive-going, both the '0' and 'pulse' led's will be on. If the pulse is negative-going, the '1' and 'pulse' led's will be on.

Table 2, variations in manufacturer

Check the listing in Table 2. It shows some variations in the 555 manufacturing process by two different manufacturers, National Semiconductor and Signetics Corporation. Since there are other manufacturers then those two I suggest when you build a circuit to stick with the particular 555 model they specify in the schematic. Meaning, the LM555 and NE555 have minor differences the way they operate. In general you can use one for the other. However, on some occasion, the NE555 will not work in your circuit but the LM555 does and vice versa. So, when a certain schematic calls for a NE555, uA555, LM555 or a MC1455 try to obtain that specific type if things don't work out.

The absolute maximum ratings (in free air) for NE/SA/SE types are:

                     Vcc, supply voltage: 18V
Input voltage (CONT, RESET, THRES, TRIG): Vcc
                          Output current: 225mA (approx)
          Operating free-air temp. range: NE555...........   0°C - 70°C
                                          SA555........... -40°C - 85°C
                                          SE555, SE555C... -55°C - 125°C
               Storage temperature range: -65°C - 150°C
Case temperature for 60sec. (FK package): 260°C

Suggested Reading:

1. 555 Timer IC Circuits. Forrest M. Mims III, Engineer's Mini Notebook. Radio Shack Cat. No: 62-5010.
"Create & experiment with pulse generators, oscillators, and time delays."

2. IC Timer Cookbook. Walter G. Jung. Published by Howard W. Sams & Co., Inc. ISBN: 0-672-21932-8.
"A reference 'must' for hobby, technicians, and engineers."

3. The 555 Timer Applications Sourcebook. Howard M. Berlin. Published by Sams Inc. ISBN: 0-672-21538-1.
"Learn how to connect the 555, perform 17 simple experiments."

4. Other good 555 timer/oscillator tutorials can be found here also:

            Williams Lab
            LM555 and LM556 Timer Circuits


  1. http://www.sentex.net/~mec1995/gadgets/555/555.html

    by Tony van Roon

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