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Integrated Circuits

Integrated circuits (ICs) are small wafers of normally silicon which contain multiple transistors, diodes, resistors, and capacitors which allow it to perform a particular function. ICs can have billions of transistors in the area of about a human thumbnail, or 25 million transistors in a single square millimetre.

ICs are often called "chips" (eg, silicon chips, microchips).

555 timer

The 555 timer IC is a very useful and versatile IC which can perform various signal conversion and timing operations. It has 8 pins.

It contains 25 transistors, 2 diodes and 15 resistors, and was designed in 1971. It has been the most popular IC ever since. Around 2003 there were over a billion 555 timers made each year. The 555 is very inexpensive, less than 5 cents each in bulk.

The 555 accepts an input signal which might be pulsed or an analogue signal, and delivers a pulsed or steady output signal. The 555 has three modes of operation, depending on what external components are connected to it.

  • Astable. Provides a pulsed output signal.

    • Usage example – electronic oscillator, which produces a steady stream of regular pulses (with no external input signal). This could be used to trigger an LED or flashgun for strobe photography, or simply flash two LEDs.
    • Usage example – analogue to digital converter (ADC), providing a pulse width modulated (PWM) output. The duty cycle of the output pulses is related to the strength of the analogue input cycle. This could be used to digitise an analogue audio signal or sensor data, such as a thermocouple used to measure temperature.
    • Usage example – electronic speed control (ESC). Produce pulses which vary from 1 ms to 2 ms, at 50 Hz (20 ms repeat).
  • Monostable. A timed output. A single input pulse will result in a long steady output pulse, of anything up to 5 minutes.

    • Usage example – timer. Power can be provided for a certain length of time (eg, security lighting, car electric windows) or withheld for a certain amount of time (eg, closing an electric garage door or arming a security system after an exit delay).
    • Usage example – a timed repetitive action. After a delay, the output pulse can be used to trigger the cycle again (eg, water spray every few minutes).
    • Usage example – switching. Debouncing switches, touch switches.
    • Note that longer output durations than 5 minutes cannot be accurately timed unless extra components are used.
  • Bistable. An input pulse turns it on, a reset pulse turns it off.

    • Usage example – Schmitt trigger. This provides a hysteresis loop, where the output turns on when the input level increases above a certain level, but turns off only when the input drops below a lower value. This is useful for removing noise from input signals, such as debouncing latched switching, or filtering high frequency noise from audio signals.
    • Usage example – single momentary button for toggle switching.

The 555 will operate on 4.5-16 V, and the output can be up to 200 mA (source or sink), meaning it can operate small relays directly (and turn them on or off when the output is high). The 555 can operate at 500 kHz (or up to up to 1 MHz if not wanting temperature stability).

4017 decade counter

This IC has the job of counting to 10. It has 16 pins, ten of which are the "decade" output pins.

The 4017 will work 3.0-18 V and can provide up to 10 mA on its output pins. It can operate up to 5 MHz when run on 5 V. It needs a clock input, for which a 555 timer IC is ideal.

Some uses of a 4017 decade counter:

  • Counting.
    • Counting the output pulses of a 555 timer to provide longer accurate timing operations.
  • LED flashing.
    • Usage example – 10 LEDs can be arranged in a circle, operated by a single 4017, to provide a continuous loop of chasing LEDs. Using two 4017s can provide up to 81 chasing LEDs. Combining with diodes can create a back-and-forth "scanning" row of LEDs – 6 LEDs with a single 4017, up to 41 LEDs with two 4017s.
  • Customisable doorbell tune.
    • Combined with a 555 timer, each of the 4017 output pins is attached to a variable resistor, providing a way of programming a ten note tune for a doorbell. Add another 555 and it can be set to play the whole 10 note tune instead of just playing while the doorbell is pressed.