Electronics Components: Double Up with the 556 Dual Timer

It turns out that there are many uses for two (or more) 555 timers in a single electronic circuit — useful enough that you can get two 555 timers in a single chip, called the 556 dual-timer chip. The 556 dual-timer chip comes in a 14-pin DIP package. The two 555 timers share a common supply and ground pin.

The remaining 12 pins are allocated to the inputs and outputs of the individual 555 timers. Here are the pin connections for each of the 555 timers in a 556 dual-timer chip. As a bonus, the pinouts for a standard 555 timer chip are included.

Function 555 Timer 556 First Timer 556 Second Timer
Ground 1 7 7
Trigger 2 6 8
Output 3 5 9
Reset 4 4 10
Control 5 3 11
Threshold 6 2 12
Discharge 7 1 13
Vcc 8 14 14

One common way to use a 556 dual timer is to connect both 555 circuits in monostable (one-shot) mode, with the output pin from the first 555 timer connected to the trigger pin of the second 555 timer. Then, when the output of the first timer goes low, the second timer is triggered.

You can connect as many 555 timers as you want in this way, with each timer’s output connected to the next timer’s trigger so that the timers work in sequence, one after the other.

For example, consider a cascaded timer circuit that uses two separate 555 timer chips. In this circuit, both of the 555 timer chips are configured in monostable mode. The time interval for the first 555 is controlled by R1 and C1. For the second 555, the interval is controlled by R2 and C2. You can choose whatever values you want for these components to achieve whatever time intervals suit your fancy.

The first 555 chip is triggered when SW1 is depressed, taking pin 2 to ground. This takes the output on pin 3 high, which lights LED1. Notice, however, that pin 3 of the first 555 is connected through a small capacitor to the trigger input of the second 555.

As soon as the time interval expires on the first 555, its output goes low, which turns off LED1 and at the same time triggers the second 555, which in turn lights up LED2. LED2 stays lit until C2 charges, and then it goes out. The circuit then waits to be triggered again by a press of the switch.

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This same circuit can be implemented using a single 556 dual-timer chip. This schematic is nearly identical to the schematic , but with a few important differences:

  • The two 555 timer circuits are designated as 556 (1) and 556 (2) to indicate that these timer circuits are part of a single 556 dual-timer chip.

  • The pin numbers indicate the pin assignments for the two timer circuits of a 556 instead of the pin assignments for a 555.

  • The second timer circuit doesn't show a supply or ground connection. That’s because the two timer circuits share a common supply and ground connection, which is shown connected to the first timer.

    image1.jpg

Although it's convenient to show the two halves of a 556 dual timer as separate components in a schematic diagram, you can show the 556 as a single component if you wish. The only difference is the way the schematic depicts the two sections of the 556 dual-timer chip.

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When you draw a 556 as a single component, it’s helpful to draw the connections for the two timers on opposite sides of the component.

Because it’s difficult to keep track of which pin is which in a 556, it’s helpful to label the function of each pin as in the schematic.

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