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USING THE 555


A 555 can be wired:
1. As a TIMER (monostable operation - also called a DELAY),
2. As an OSCILLATOR (also called a MULTIVIBRATOR - or astable operation)
3. As a ONE-SHOT (also called monostable operation).
The 555 IC is an extremely popular IC. It is simple to use and very rugged.
It comes in a single, dual or quad package with part numbers such as LM555, NE555, LM556, NE556. It is ideal for astable (free-running) oscillators as well as the one-shot monostable mode.
The 555 can be triggered and reset on falling waveforms and the output can source or sink up to 200mA. The HIGH output is about 1.7v less than supply. The NE555 operates 3v - 16v DC.
Maximum operating frequency is 500kHz.
THE 7555

7555 is a CMOS version of the 555. It is exactly the same as the 555 but consumes less power. The 555 consumes 10mA, while the 7555 consumes 80uA (1/120th). The CMOS version comes with different identifications according to the manufacturer.
LMC555 or LM555CN is made by National Semiconductors, TLC555 is made by Texas Instruments, ICM7555 is supplied by Philips, ZSCT1555 comes from Zetex and ICM7555 is made by Maxim. The main feature to note is the inclusion of the number "7" or the letter "C" to identify the CMOS version.
They use less power than the older (555, NE555, LM555) versions and don't require a capacitor on the control pin. Although pin and functionally compatible, the component values differ between the low-power CMOS and older versions.
The Exar XR-L555 timer is a micro-power version of the standard 555 offering a direct, pin-for-pin substitute with the advantage of lower power operation. It is capable of operation from 2.7v to 18v. At 5v, the L555 will consume about 900 microwatts, making it ideally suitable for battery operated circuits. The internal schematic of the L555 is similar to the standard 555 but with current-spiking filtering, lower output drive capability, higher nodal impedances, and better noise reduction system.

USING THE 7555

The ICM7555 is a CMOS timer providing significantly improved performance over the standard NE/SE555 timer, while at the same time being a direct replacement in most applications.
Improved parameters include low supply current, wide operating supply voltage range, low THRESHOLD, TRIGGER, and RESET currents, no crow-barring of the supply current during output transitions, higher frequency performance and no requirement to decouple CONTROL VOLTAGE for stable operation.
The ICM7555 is a stable controller capable of producing accurate time delays or frequencies.
In the one-shot mode, the pulse width of each duration is precisely controlled by one external resistor and capacitor.
For astable operation as an oscillator, the free-running frequency and the duty cycle are both accurately controlled by two external resistors and one capacitor. Unlike the bipolar 555 device, the CONTROL VOLTAGE pin does not have to be decoupled with a capacitor.
The output can source or sink currents large enough to drive TTL loads or provide minimal offsets to drive CMOS loads.
Maximum output current 50 - 80mA.

  • Exact equivalent in most applications for NE/SE555
  • Low supply current: 80µA (typical)
  • Extremely low trigger, threshold, and reset currents: 20pA (typical)
  • High-speed operation: 500kHz guaranteed
  • Wide operating supply voltage: 3v to 16v
  • Normal reset function. No crow-barring of supply during output transition
  • Can be used with higher-impedance timing elements than the bipolar 555 for longer time constants
  • Timing from microseconds to hours
  • Operates in both astable and monostable modes
  • Adjustable duty cycle
  • Output source/sink driver can drive TTL/CMOS.
  • Maximum output current 50 - 80mA.
  • Typical temperature stability of 0.005%/°C at 25°C
  • Rail-to-rail outputs

An improvement on the CMOS 7555 is the ZSCT1555 from Zetex. It is guaranteed to work down to to 0.9 volts with bipolar technology. It has been designed for portable applications, by offering single battery cell operation. (See end of P3 for a technician's difficulty with getting this chip to oscillate.)
It provides the same precision timing capabilities as its predecessors, (the 555 and 7555) it has the same 8 legged pin-out. With the simple adjustment of external passive components to set the frequency, the device's function is just the same, whether it be generating accurate time delays or oscillations.
Assuming a 5v supply, a typical CMOS part draws 170uA
while the new timer pulls 140uA, and at 1.5v just 75uA.

555 Vs 7555

The choice between the standard 555 and CMOS version (7555) or ZSCT1555 will depend on cost, availability, load current required and frequency of operation. It will mainly come down to battery or mains operation for the project.
Normally, when we change from a TTL chip to a CMOS chip, the component values change by a factor of 10x or 100x. This is because the TTL chips are very low impedance and CMOS is very high impedance.
But if a 555 is substituted for a CMOS version, the timing components remain the SAME!
This is very convenient. Chips can be substituted without having to alter the surrounding circuitry. The only change will be the current consumption of the chip. In general, the consumption will reduce from about 10mA to approx 0.5mA. (A LED Voltmeter circuit made the following circuit-current comparison: using 555 = 7mA, using 7555 = 0.35mA). This is typical of the current-saving of a CMOS version.
This article covers most types and provides a number of comparisons and substitutions.
A typical 7555 circuit is shown below:

Note the need for the driver transistor in the circuit above, as the 7555 has an output capability of about 50mA.