THE 556

THE 556
The 556 is a DUAL 555. It contains two identical 555 timer circuits. The NE556/SE556 timers can be directly replaced by the CMOS types MC3456/MC3556.

The following three pin-outs identify the 556 dual timer IC and the function of each pin.

Discharge is "in-phase" with the Output. (both are low at the same time)
Threshold detects the high voltage on the capacitor and makes Discharge and Output go LOW
Trigger detect the low voltage on the capacitor and makes Discharge and Output go HIGH

Output HIGH (mS) Output LOW (mS) Frequency (kHz)

Calculate the Frequency, HIGH-interval and LOW-interval
for a 555 in ASTABLE mode. See answer below for Monostable (Delay):

Delay Sec

HIGH Interval (T₁) = 0.693 x (R₁+R₂) x C
LOW Interval (T₂) = 0.693 x R₂ x C
Frequency = 1.44 / ( (R₁+R₂+R₂) x C)

HOW TO USE THE CALCULATOR

Enter values for R₁, R₂, and C and press Calculate to determine HIGH interval and LOW interval. For example, a 10k (R₁), 100k (R₂) and 100n will produce output time intervals of 7.62mS HIGH and 6.93mS LOW. The frequency will be about 70Hz and a Delay of 0.01sec. R₁ should be greater than 1k and C should be greater than 1n.

SUBSTITUTING A 555

A 555 (and the 556 varieties) can be replaced by low-power 555's such as TLC555, LMC555, ICM7555. This has been covered above.

In this section we will show how to replace any of the 555 or 7555 devices with a building block called a SCHMITT TRIGGER. The Schmitt Trigger chip we suggest is a 74c14 (40106 -CD 40106). This chip contains six Schmitt Triggers. It allows up to 6 building blocks to be created, similar to the capabilities of a 555.

This is a much-more economical and professional way to designing a circuit and two other very important features are also provided.

The Schmitt Trigger consumes less current and battery designs can be created.
A Schmitt Trigger does not put noise on the power rails of a project and it can be used with other digital blocks without creating interference problems.

Six gates in a single hex Schmitt trigger chip allows the designer to produce 6 different building blocks and quite complex circuits can be produced.
The type of 555 circuit we are suggesting be replaced with a Schmitt design is one that meets one of more of the following criteria:
1. A design that needs to be upgraded and improved in "professionalism."
2. A design that needs to be reduced in quiescent current,
3. A design that uses more than one 555
4. A design that employs 555 IC's with digital IC's.
A simple 555 design for a car, for example, does not need to be converted.

THE 555 OSCILLATOR
The following diagrams show a free-running 555 oscillator and its Schmitt Trigger equivalent.
The circuit can be called an OSCILLATOR, SQUARE-WAVE OSCILLATOR or FREE-RUNNING OSCILLATOR.
The 555 can sink or source 200mA and the two diagrams show this:

The only difference between the two circuits is the Schmitt version will draw about 10mA -15mA less.
The 555 draws about 10mA for its internal operation and about 1mA - 5mA will be "wasted" through R2.
If the load is less than 25mA, the following circuits can be used:

The output of a single Schmitt Oscillator will drive a load up to 25mA, depending on the frequency of oscillation and the voltage of the supply. As the voltage decreases, the load current reduces. At 5v, the load will be a maximum of 10mA.
As the load current increases, the output will not rise to 66% of rail voltage and the oscillator will "freeze."

PULSE GENERATOR

Another name for "oscillator" is PULSE GENERATOR. The following circuit shows a 555 wired as a square-wave oscillator called a MULTIVIBRATOR. The output waveform is adjustable and is ideal for injecting into RF and IF stages. The square-wave is rich in harmonics and will pass through both RF and IF stages to produce a tone or "buzz" in the speaker.

CHANGING THE MARK-SPACE RATIO

The MARK and SPACE are the HIGH and LOW values of a waveform. When a waveform is HIGH, it is called the MARK.

The diagram above shows MARK and SPACE durations of different lengths. Marks and spaces can be any length and can change during the production of a waveform. If the length of the mark is equal to the space, the waveform is said to have a 50:50 Mark:Space ratio, as shown below:

A waveform with a 50:50 Mark:Space ratio is produced by a 555 when the top resistor (called the DISCHARGE resistor) is very small compared to the TIMING resistor. This is shown in the diagram below:

To increase the MARK, the Discharge resistor must be LARGE compared to the Timing resistor.

To increase the SPACE, a diode is needed as shown in the diagram below:

The MARK:SPACE ratio can be adjusted without altering the frequency by connecting two diodes as shown in the diagram below:

GATING THE OSCILLATOR

The 555 oscillator can be turned on and off via a control line. This is called "Gating the Oscillator" or "Controlling the Oscillator."
When designing this type of circuit, two things need to be considered:
1. The oscillator to be switched off with the output HIGH
2. The oscillator to be switched off with the output LOW
The next consideration is:
3. The oscillator (block) to be switched off
4. The oscillator (block) to remain in circuit.
There is an enormous difference between these designs. The main difference is the current consumption of the load, but the actual consumption of the chip can also be important.
Take for example, these two circuits:

In the first circuit, the key is in the output and the chip draws current all the time. If the project is battery operated, it will need an on-off switch. The second circuit uses the key as the switch and the circuit will not need a switch. The difference is only 10mA but if the first circuit is left on, the battery will

The next circuit shows one way to turn off a 555 after a period of activation:

The only problem with this circuit is the gradual lowering in volume as the electrolytic discharges. The 1,000u to 4700u determines the length of time the circuit is activated AFTER the Bell-Push is pressed. The circuit drops to zero current (the only current is the leakage of the 1,000u electrolytic).

In the following circuit the first 555 gates the second 555.

The second 555 is not turned off. The circuit inside both 555's are always drawing current.
It is not practical to "turn off" a 555 as shown in the next diagram:

The output of a 555 is 1.7v less than rail voltage. This means the second 555 is receiving 10.3v from the output line of the first 555 if the rail voltage is 12v. The maximum output of the second 555 will be 10.3 - 1.7 = 8.6v This may be too low for many output devices and the result may be disappointing.
The Schmitt Trigger can be gated too.
The first point to note is the hex Schmitt trigger IC contains 6 identical gates and the chip is normally permanently connected to the supply rail. If any of the unused inputs are tied HIGH, the particular gate draws very little current (less than 1uA), making the total for the chip about 6uA.
There are two ways to GATE a Schmitt Trigger and prevent it from oscillating.
The diagrams below show a Schmitt Trigger being gated so that the output is:

1. LOW,
2. HIGH.

Mouseover the animations below and see how the GATING LINE inhibits the oscillator:

Mouse-over to INHIBIT the Left Oscillator

For the left circuit, if the gating diode is taken HIGH, the capacitor charges quickly. This inhibits the operation of the oscillator and the output goes LOW. If a load is connected to the output of this gate, it will not be driven and the gate will consume the least current.
For the right circuit, when the gating diode is taken HIGH it does not have any effect on the operation of the circuit and the oscillator continues to operate.

Mouse-over to INHIBIT the Right Oscillator

For the left circuit, if the gating diode is taken LOW, it does not have any effect on the operation of the circuit and the oscillator continues to operate.
For the right circuit, if the gating diode is taken LOW, the capacitor is discharged and the oscillator is INHIBITED. The output goes HIGH and the load will be driven. The circuit will draw maximum current.

If NO LOAD is connected to the output, an inhibited gate will draw more current than when it is oscillating. Both arrangements will draw a similar current when inhibited. The current taken will be about 1uA for the gate plus the current through R.

THE 555 AS A DELAY

The 555 can be used as a timer up to 10 minutes. This circuit is also called a DELAY.
To start timing, the START button is pressed briefly and the output of the chip goes LOW. At the expiration of 10 minutes, the output goes HIGH and the red LED illuminates.
A simple application may be for a cooking operation in a shop.
If a product needs to be cooked or heated etc, the button can be pressed and the LED illuminates when the time has expired.
When calculating the time-duration for the circuit above, the capacitor charges from 0v to 2/3 rail voltage.

DRIVING HIGH-CURRENT LOADS

The output drive-current for a 555 is 200mA maximum. The output voltage is 1.7v less than rail voltage.
A driver transistor can be connected to the output pin to improve the output current to 1amp (or more) and deliver an output voltage that is near rail voltage. Globes are a typical example of a high-current load. They require up to 6 times the normal current when starting. This is due to the cold filament having a very low resistance. The same applies to motors. They have a high start-up current requirement.
Any driver transistor can be fitted as shown in the diagram below:

Use a driver transistor for loads greater than 200mA

The CMOS 7555 has an output current capability of 50mA and will need a driver transistor for currents above 80mA.