Dual Astable 555 Timer

The 555 which comes in a number of technology variants (Bipolar & CMOS), single, dual elements and different package styles (SMD & DIL), and finds many uses in a variety of applications from timers, pulse generation and oscillators.

All of these applications have one thing in common and that is a single output on pin 3 which can both source and sink current.

A typical use for the 555 is as an Astable Oscillator with the 2 resistors version being the most referenced allowing duty cycle adjusting by the ratio of these 2 resistors.
However, there is an alternative and simpler single resistor version if a ~50% duty cycle will suffice. By utilising this simpler version we can turn a single output Astable into a two output Astable Oscillator. So how is this achieved.

555 Timer

LED's x 2

220R resistors x 2

470k resistor.

1uF/16V Electrolytic Capacitor

100nF ceramic Capacitor

20 AWG Enamelled copper wire

22 AWG Enamelled copper wire

Needle files or other tools to remove the enamel to enable soldering.

With the circuit configured as a single resistor Astable.

In this configuration the Control Pin is connected to a capacitor to decouple this pin from external noise, this pin can be used to vary the threshold voltage externally.

The Reset pin is connected to V+.

Trigger and Threshold are connected together and these are connected to one input each of the 2 internal comparators.

Each comparator has a 2nd input which is connected to one of the tap points of the three 5k resistors in series. The negative input of one comparator is set at 2/3 V+ whilst the positive input of the other comparator is set at 1/3 V+.

The midpoint of the capacitor, resistor (CR), network connects to the two connected points of Trigger & Threshold with the resistor connected to the output.

If we start with the assumption that the output is High (it could equally have started Low), the capacitor begins to charge via the resistor until the voltage reaches 2/3 V+ this causes the Flip Flop to change state and switches the output Low. The capacitor discharges via the resistor until the voltage reaches 1/3V+ causing the Flip Flop to change state and the output goes High.

This process repeats indefinitely as long as power is maintained.

Now there is one pin left Discharge, this is an open collector transistor; in the typical 2 resistor configuration this discharges the capacitor via RB, when the transistor switches on.

But in the single resistor configuration this pin is not connected, however as it is internally connected to the output of the Flip Flop its available for use. When the output of the Flip Flop goes High the transistor switches on and acts as a current sink.

We can make use of this Discharge pin if an LED is connected to this pin via a resistor to V+ it will light up.

The Output pin will be in the opposite state and an LED connected to 0V via a resistor will be off.

However, unlike the Discharge pin the Output pin can both source and sink current meaning that an LED with resistor can be connected from Output to V+ or 0V.

Therefore we now have 2 LED's switching in alternate states without requiring additional external switching components and making use of what would be an otherwise unused pin.

Note: That the Discharge transistor has a maximum current rating ~1/4 that of the Output pin.

Additionally, device currents will vary dependent on technology, package and design, therefore consult the manufacturers datasheet for device specific details.

To illustrate this additional output in action I assembled the single resistor Astable configuration.

But instead of using a circuit board I built it using wire both as the support and the circuit but at the same time creating a freestanding form both for support and display.

The main structure of the circuit would be made from enamelled copper wire formed into the shape of a cube.

The cube is constructed of enamelled copper wire as this can easily be bent into shape whilst at the same time being insulated.

This insulation will allow the components to be isolated preventing shorts.

The cube will be make from one continuous piece of wire with no additional cuts and will simply be bent into shape.

To aid in shaping the wire frame a 3d printed cube was used as a template.

Once completed the corners will be soldered to add strength and rigidity enabling the components to be mounted to the framework.

The cube will act as the 0V for the circuit.

Connections not going to 0V will be made with enamelled copper wire links.

The Timer is mounted in the centre of the outer face of one of the sides.

Components are then mounted from the centre to the outer edge.

Contact points on the outer edge will require that the enamel is removed to expose the copper to allow soldering.

Removal of the enamel can be achieved using a sharp blade, a needle file or electric hand drill with sanding tool.

Once the enamel is removed, tin the exposed area of both joints and solder the two together.

Attaching two corners first, (pin 1 - 0V, and pin 5 - control), helps to stabilise the timer making it easier to attach the remaining components.

Pin1 is a direct connection to the frame whilst pin 5 is connected via the decoupling capacitor.

Once these are attached it makes it easier to fit the other components.

These consist of the timing capacitor to pin 6 (Threshold), wire link from 2 (Trigger) to 6, timing resistor from 2 to 3 (Output), Reset from 4 to 8 (V+), LED plus resistor from 1 (GND), to 3 and LED plus resistors from 7 (Discharge), to 8.

Finally to connect the supplies, 0V to the framework and V+ to pin 8 or 4.

Hope you found this interesting.

On a final note:

Regarding how the name for the 555 came about and Hans Camenzind, further details can be found at Semiconductormuseum

Note: I have no affiliation with this site.