After finishing the X-Terminator, which worked exactly as described, I began to think of additions that could be made to it – two in particular, which I thought might be useful.
This extract from the circuit diagram shows the one slight change I had made from the original circuit of the Cacophonator – replacing the latching switch and the momentary push button with a single toggle switch, latching in one direction, momentary in the other:
It also shows the very big capacitor, C7, which enables the X-Terminator to keep sounding for some time after being charged by a quick press of the momentary switch connected to +V.
First of all, the original description mentions that ‘the discharge period for C7 [i.e. how long it keeps sounding] is affected by the load at the audio output, which should therefore be a high impedance in order to obtain this period.’
Rather than rely on the vagaries of whichever unit the X-Terminator might be plugged into, I decided to provide it with its own high impedance circuit to maximise the period for which it would sound. This circuit would be a completely separate unit, connected to a different power supply, so as not to interfere with the working of the X-Terminator.
Secondly, I thought it would be handy to have a method of automatically retriggering the device after a set interval: a variable, but fairly long interval – up to a couple of minutes – to give it time to finish ‘winding down’.
In order to provide an external method of doing this, I added a socket in parallel with the switch, which would connect to the new unit:
I was intending to use a suitable Dr Who-related case for the project, but couldn’t find a model of K9 which was big enough. The only K9 I could get hold of was very small, and not really of sufficient size to contain any worthwhile circuitry. So, in the end I had to be satisfied with adding a second 2.5mm socket which would allow a button on the small K9 to operate another of the samples in the Dr Who talking keyring which had been incorporated into the X-Terminator case (the Dalek).
New sockets on the back of the X-Terminator
The button was a tiny tactile switch connected to to a 2-way cable which terminated in a 2.5mm mono plug. This fitted into a new socket on the back of the Dalek which was wired to the keyring PCB.
The K9 model itself was not only small – approximately 7cm from nose to tail – but rather too light, so I added some weights to give it a bit more body. These could be useful in other situations where a case might not be heavy enough to resist being dragged about.
Originally intended for tyre balancing, these were either 5g or 10g and were sticky on the back, so they could be cut from the strip and easily fixed in place. I used one of each in the base of the K9 before gluing it back together:
When the button is pressed, the response ‘Affirmative’ is given. This, of course, comes from the speaker in the base of the Dalek.
When it came to the audio section of the unit, I had a clear plan, which would use a quad op-amp: at first an LM324, of which I had several lying about, to be replaced by a TL074 if the audio quality of the LM324 wasn’t good enough, as I had read might be the case. These two chips have exactly the same pinout, so a swap would be perfectly possible once the circuit was up and running.
The reason I wanted a quad op amp was because the first would be used as a high impedence buffer (or non-amplifying amplifier), while the other three would form a phase-shift-type stereo simulator. The circuit looked like this:
I connected it up, but for some reason it wouldn’t work, the output was mono and the cacophonator would still not continue sounding for more than about 15-20 seconds. I removed the battery to do some further testing and . . . the circuit worked perfectly: a noticeably stereo sound and up to a minute of cacophony while the big capacitor slowly discharged . . .
I tried removing the LM324, but the sound reverted to mono . . . I noticed the input signal and 0v leads were connected the wrong way round so I changed them . . . but the sound reverted to mono . . . so I just left the chip in and removed the power leads. I really have no idea why it worked – at least to an extent – in this condition: maybe the collection of components forms a kind of filter network, affecting one channel differently to the other . . . however, it worked, so that was fine by me. Perhaps I’ll go back to it later and try and sort it out properly . . .
So much for the audio section. Turning to the auto-trigger section, the requirement here was for a circuit which would briefly close the switch to charge up the big capacitor, wait for a certain length of time for the resultant cacophony to subside, then repeat the process, and continue repeating at intervals until turned off. The sonic results of this would be more interesting than leaving the switch permanently on.
As previously mentioned, this could only be done by a circuit kept separate from the X-Terminator circuit if the latter was to run as intended. So I decided to use an electronic switch, one of the four such switches in a 4066 integrated circuit:
The two sides of the charging switch – brought into the unit via a 2.5mm mono socket – would be connected to, for example, pins 1 and 2 in the above pinout diagram. Normally, pins 1 and 2 remain unconnected, as their control pin, pin 13, is held low – i.e. connected to 0v by a 100k resistor. The lines to pins 1 and 2 can be connected together, like throwing a switch, by inputting a +V pulse into pin 13.
In the final version I used pins 10 and 11, rather than 1 and 2, with the control signal going into pin 12, but the effect was exactly the same: a +V pulse into pin 12 would connect the switch and trigger a new cycle of cacophony from the X-Terminator.
I planned to add a push-button on the top of the unit and a 6.35mm mono socket for a momentary footswitch. These would both be capable of sending a +V pulse into pin 13 and trigger the charge switch as an alternative to the switch on the X-Terminator itself.
However, my main objective was to have this done automatically.
The way to do this, I reasoned, was to use a very common timer or oscillator chip, the 555. A 555 set up in astable (‘continuous’) mode, but running very, very slowly, with a variable control to go from 0v to +V at intervals of between 10 to 15 seconds minimum and one or two minutes maximum. In addition, the ratio between the ‘on’ (+V) time and ‘off’ (0v) time would be set so that it was mostly off, and turn on briefly for a suitably short length of time to operate the trigger.
The typical connections for a 555 in astable mode are like this:
The frequency at which the circuit oscillates (goes from 0v to +v and +v to 0v) is determined by the three components Ra, Rb and C. The way to allow the circuit to oscillate at a range of frequencies is to put a potentiometer in place of Rb.
The version of the circuit I used has an extra component which isn’t normally necessary. This is the diode in parallel with Rb, between pins 6 and 7. The purpose of this is to allow the time the circuit spends at +v to be much shorter than the time it spends at 0v. The effect of this will be that if it’s oscillating at a very slow rate – say once a minute – there will just be a short pulse at the beginning of the minute to trigger the X-Terminator, and then the trigger signal will remain off until the next minute begins.
A preset variable resistor in place of Ra can be used to increase or decrease the length of the pulse, to make sure the X-Terminator is reliably triggered.
There are numerous places on the internet to find out more about the very useful little 555. Some of my favourites are www.555-timer-circuits.com, www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html and www.electroschematics.com/555-circuits/.
As for the case itself, this ended up less as Dr Who, more like ORAC from Blake’s 7. Not very like ORAC, really . . . but it was transparent and had flashing lights in it, which was near enough for me!
While researching a suitable 555 timer circuit for the automatic trigger, I came across (here: http://www.instructables.com/id/LED-cube-using-4020-Ic-and-555-IC/) a circuit for turning 9 groups of 3 LEDs on and off in sequence, using another 555 and a 4020, a counter with up to 14 stages.
The 27 LEDs are typically arranged in a cube shape, and I thought something like this would be an attractive indicator that the automatic trigger was on. So along with the circuitry I began supergluing LEDs on the inside of the transparent plastic box.
Just to make it different – and to cut down slightly on the number of LEDs – I used 4 red-blue bi-colour LEDs. These have 3 legs, so each would have to be last in a series of 3, enabling each of the two outer (anode) legs to be the end of a series of 3, and the centre (cathode) leg to be connected to ground.
So the timer/flashing light part of the circuit needed two 555’s, a 4020 and a 4066. Rather than use two 555’s, I used a 556 – a dual 555 – and the final circuit looked like this:
Wiring all the LEDs together was quite tricky, but gradually it all began to come together . . .
. . . and I was delighted when I connected a battery and the lights began to flash!
I needed to protect some parts with bubble-wrap, to avoid short circuits, but when the lid was back on, and the trigger and audio were connected to the X-Terminator, it operated exactly as intended.
This has added some extra and very useful functionality to the original ‘Cacophonator’ circuit.
The finished ORAC:
and with the external footswitch attached:
Incidentally, it was only shortly before I began on this series of posts that the inventor of the 555 died in Los Altos, California.
Born in 1934, Hans Camenzind came to the United States from his native Switzerland in 1960. At first he worked for various companies in the newly developing semiconductor industry, such as Transitron, Tyco Semiconductor, and Signetics, then in 1971 formed his own company, InterDesign, specializing in semi-custom integrated circuit design.
It was there, working under a contract with Signetics, that he invented the 555 timer, one of the most successful chips in the history of the semiconductor industry, and one which is still widely used today. Versions of the device have been or are still made by dozens of major semiconductor vendors, including Texas Instruments, Intersil, Maxim, Avago, Exar, Fairchild, NXP and STMicroelectronics.
Camenzind also introduced the idea of phase-locked loop to design and invented the first class D amplifier. Altogether he was named as inventor in 20 patents. He was a prolific author with interests as diverse as electronics textbooks and the history of the semiconductor industry, to a book on God and religion. He also lectured at the University of Santa Clara.
His last book, Much Ado About Almost Nothing, subtitled A history of electricity and electronics, written for the non-expert, sounds quite interesting. You can read about it at www.historyofelectronics.com.