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555 Timer

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What is the 555 timer?

The 555 timer is one of the most remarkable integrated circuits ever developed. It comes in a single or dual package and even low power cmos versions exist - ICM7555. Common part numbers are LM555, NE555, LM556, NE556. The 555 timer consists of two voltage comparators, a bi-stable flip flop, a discharge transistor, and a resistor divider network.

I am particularly indebted to Philips Components and Semiconductors Australia for their most generous assistance in giving me access to material presented on this page.

Philips describe their 555 monlithic timing circuit as a "highly stable controller capable of producing accurate time delays, or oscillation. In the time delay mode of operation, the time is precisely controlled by one external resistor and capacitor. For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms, and the output structure can source or sink up to 200mA."

What are the 555 timer applications?

Applications include precision timing, pulse generation, sequential timing, time delay generation and pulse width modulation (PWM).

Pin configurations of the 555 timer

Here are the pin configurations of the 555 timer in figure 1 below.

Figure 1 - 555 timer pin configurations

Pin Functions - 8 pin package

Ground (Pin 1)

Not surprising this pin is connected directly to ground.

Trigger (Pin 2)

This pin is the input to the lower comparator and is used to set the latch, which in turn causes the output to go high.

Output (Pin 3)

Output high is about 1.7V less than supply. Output high is capable of Isource  up to 200mA while output low is capable of Isink  up to 200mA.

Reset (Pin 4)

This is used to reset the latch and return the output to a low state. The reset is an overriding function. When not used connect to V+.

Control (Pin 5)

Allows access to the 2/3V+ voltage divider point when the 555 timer is used in voltage control mode. When not used connect to ground through a 0.01 uF capacitor.

Threshold (Pin 6)

This is an input to the upper comparator. See data sheet for comprehensive explanation.

Discharge (Pin 7)

This is the open collector to Q14 in figure 4 below. See data sheet for comprehensive explanation.

V+ (Pin 8)

This connects to Vcc and the Philips databook states the ICM7555 cmos version operates 3V - 16V DC while the NE555 version is 3V - 16V DC. Note comments about effective supply filtering and bypassing this pin below under "General considerations with using a 555 timer"

555 timer in astable operation

When configured as an oscillator the 555 timer is configured as in figure 2 below. This is the free running mode and the trigger is tied to the threshold pin. At power-up, the capacitor is discharged, holding the trigger low. This triggers the timer, which establishes the capacitor charge path through Ra and Rb. When the capacitor reaches the threshold level of 2/3 Vcc, the output drops low and the discharge transistor turns on.

The timing capacitor now discharges through Rb. When the capacitor voltage drops to 1/3 Vcc, the trigger comparator trips, automatically retriggering the timer, creating an oscillator whose frequency is determined by the formula in figure 2.

Figure 2 - 555 timer in astable operation

There are difficulties with duty cycle here and I will deal with them below. It should also be noted that a minimum value of 3K should be used for Rb.

Figure 2a - modified duty cycle in astable operation

Here two signal diodes (1N914 types) have been added. This circuit is best used at Vcc = 15V.

555 timer in monostable operation

Another popular application for the 555 timer is the monostable mode (one shot) which requires only two external components, Ra and C in figure 3 below. Time period is determined by 1.1 X Ra C.

Figure 3 - 555 timer in monostable operation

General considerations with using a 555 timer

Most devices will operate down to as low as 3V DC supply voltage. However correct supply filtering and bypassing is critical, a capacitor between .01 uF to 10 uF (depending upon the application) should be placed as close as possible to the 555 timer supply pin. Owing to internal design considerations the 555 timer can generate large current spikes on the supply line.

While the 555 timer will operate up to about 1 Mhz it is generally recommended it not be used beyond 500 Khz owing to temperature stability considerations.

Owing to low leakage capacitor considerations limit maximum timing periods to no more than 30 minutes.

External components when using a 555 timer

Care should be taken in selecting stable resistors and capacitors for timing components in the 555 timer. Also the data sheet should be consulted to determine maximum and minimum component values which will affect accuracy. Capacitors must be low leakage types with very low Dielectric Absorption  properties. Electrolytics and Ceramics are not especially suited to precision timing applications.

Figure 4 - equivalent schematic of a 555 timer

Very low frequency timing

There have been a few people who have written to me about problems in using the 555 timer in very low frequency applications. One particular case was at 10 Hz:

"I made an oscillator using 555 IC running in astable mode, at 10.0HZ. My problem is extreme over heating and ultimate burn out. Can you give a stable circuit as 555 to run at 10HZ without over heating and extremly stable"

My advice was to consider using another device such as the 74HC4060 "14-stage binary ripple counter with oscillator". You can use more than one device in series (cascading) for higher division numbers (with some often inconvenient gaps).

The 74HC/HCT4060 are 14-stage ripple-carry counter/dividers and oscillators with three oscillator terminals (RS, RTC and CTC ), ten buffered outputs (Q3 to Q9 and Q11 to Q13 ) and an overriding asynchronous master reset (MR).

The oscillator configuration allows design of either RC or crystal oscillator circuits. The oscillator may be replaced by an external clock signal at input RS. In this case keep the other oscillator pins (RTC and CTC ) floating.

One device is capable of dividing by Q13 (that's Philips notation for 14 as they consider Q0 as a valid first number division). If you don't understand binary division it may pay to look at my page on digital basics. You can cascade as many devices as you like although you will get some gaps in divisions available to you - see data sheet.

To achieve a highly accurate 10 Hz output I'd suggest doing this:

Figure 5 - 74HC/HCT4060 14-stage ripple-carry counter/dividers used to achieve 10 Hz output

Now if I've got it right and, it's been a long while since I did this, that schematic should work.

I've selected the Q13 output of the first device (which is actually the 14th stage) and this gives us 5242880 Hz divided by 16384 = 320 Hz. This is fed into the RS input of the second device where the output is taken from the Q4 stage which is 320 Hz divided by 32 = 10 Hz.

To some people the confusing part is Philips use the system of Q0, Q1, Q2.., Q12 and Q13. So Q13 is actually the 14th stage and Q4 is actually the 5th stage. Adding the two together, 14 + 5 = 19 (oh duh), means if we successively divided 5242880 by 2 for nineteen times we will get down to 10 Hz.

Several points to bear in mind:

• maximum clock frequency is a very generous 80+ Mhz
• you can cascade as many devices as you like
• the accuracy of your output frequency is entirely the same as your source
• this only one example - use your imagination

It's an interesting device to play around with. I've used it to play around with hee-haw sirens and other amazing yet, foolish projects.

Don't forget to consult the data sheet, print it out and look at my page on digital basics.

IC 555 Projects - E.A. Parr - Babani 141 pages

This book by E.A. Parr proves a very inexpensive source of "555 timer projects"

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555 Timer Applications Sourcebook, With Experiments

555 Timer Applications Sourcebook, With Experiments
by Howard M. Berlin

Out of print but available second hand.