Datasheets : 555 Monostable - Process Unit

What does it do?

The 555 monostable subsystem provides an output signal that is triggered high for a period of time before returning to low.

How does it operate?

Once triggered the output remains high for a short time period afterwards. This time period can be calculated:

where R is in M ohms and C is in µF. For example, if R = 100k (= 0.1M) and C = 22µF then the delay time is equal to 1.1 ´ 0.1 ´ 22 = 2.4s.

So, by choosing different component values and adjusting the variable resistor, the delay time can be varied.

The 555 timer IC works with a d.c. power supply with a voltage between 4.5V and 16V. The 555 timer is able to provide an output current of 100mA and can therefore drive low and medium current output devices directly.

There is one quite subtle point to watch with a 555. Nearly all the digital process ICs discussed on this web site are 4000 series CMOS devices. The 555 timer is not in this family and there can be problems in mixing families. In particular, a 4000 CMOS device with a supply voltage of 5V needs an input signal voltage of at least 3.5V to guarantee that it is recognised as a logic '1' or 'high'.

However, at the same supply voltage, a 555 produces an output signal voltage of typically 3.3V, and it can be as low as 2.75V. In other words, the 'high' from a 555 may not be recognised as a 'high' if it is fed to a 4000 series process subsystem.

The safest way to deal with this is to use a CMOS version of the 555 e.g. the ICM7555 (which is more expensive), or a delay unit or retriggerable delay unit – both of which use 4000 series CMOS.

Capacitor types
Ceramic disc capacitors should not be used for the timing capacitor C. They are not sufficiently stable in capacitance to operate properly for timing. Suitable capacitor types are: silver mica, mylar, polycarbonate, polystyrene, tantalum, or similar types.

Possible applications

• Sounding a warning for a short time
• Keeping a car interior light on for a few seconds after the doors have closed.

Making

Build and test the unit that will trigger the 555 astable before building the 555 astable.

Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:

• the voltage on pin 1 is low (0V);
• the voltages on pins 4 and 8 are high (the supply voltage);
• the voltage on pin 2 (the blue PCB track) goes high and low in response to the unit that provides the input signal.

Insert the IC the right way round.

Testing

Make sure that the signal going out (on the green PCB track) is low at first (while the input signal is high) and then, when the input signal goes low for a short time, the signal going out goes high for the expected time and then goes low.

Fault finding

If there is a fault, check that:

• The voltage on pin 1 is low (0V)
• The voltage on pins 4 and 8 is high (the supply voltage)

If there is a fault, check the tracks and solder joints.

Alternatives

• Delay Unit – useful if ‘spare’ gates are available but delay time is not so precise and more restricted.
• The retriggerable monostable provides a pair of simple delay units but are more expensive.
• PICs – more flexible but more expensive.

Web Links

• Manufacturer's data sheet for 555 timer IC

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