And now for the world’s most famous IC – the 555 Timer
This Signetics – Hans Camenzend IC was not just a best seller but for electronics specialists possibly the most versatile chips of it’s time. Possibly still is. What was most intriguing was that for any youngster just starting out in electronics, this was an experimenter’s dream. It was low cost and with just a handful of components one could learn one helluva lot about oscillators (astable or free-running), bistables and monostables. Of course it could be used for a lot more than just a police siren.
The 555 is known as a timer chip and a comprehensive write-up can be found on Wiki and in Circuits Today we have the story behind the making, “The History of 555 Timer IC – Story of Invention by Hans Camenzind“. Yep, we’re not going to re-invent the wheel here, too much has been said already.
Moving on to other things, why select this chip for an article in the analogue world where it is used quite often in digital circuits? For us here at AI I would propose three things immediately: (i) a signal generator and (ii) a boost switcher for low current tubes like dual triodes (iii) Phantom power supplies for condenser microphones (using a charge pump circuit).
We are not going to draw some pretty diagrams because they have been done already. TI give a very detailed description of this chip and if you are lucky you can even get a free sample. But hey, Yebo Electronics plus a host of other suppliers sell them for only R10.00 so why wait!
The Texas Instruments datasheet. [pdf]
This chip, the NE version is rated at up to 16V, the SE (military) up to 18V. Rise and fall times of output according to the datasheet for modern devices are supposedly max 1nS or 1MHz. Experimentation realises frequencies of 500kHz which are not improbable in my own tests (at lower voltages) at 12V, at 5V it is output compatible to TTL and sourcing or sinking of output currents at 200mA is not too shoddy. Output buffering is common. Is it OK to be used in SMPS boosting circuits?
The circuit shown here is a voltage pump which converts USB voltages to 12V but can easily be modified to +48V for a condenser microphone phantom supply.
Circuits showing Nixie tube displays are often driven by 555 timer with Mosfet switchers – here is one with a +180V output. Such a circuit, with a tweak here and there would be sufficient to power a dual triode such as an ECC82 or 83. (12AU7 / 12AX7). I stuck with this circuit for simplification and ease of modification or a similar circuit, using either single inductance or transformer boosting.
Although the 555 timer is a really wonderful piece of engineering readers should note that there are limitations on max time control and current sourcing / sinking. There are however successful work-arounds.