Snap Circuits Science: Digital Modulation
By Steve Schuler | September 13th 2013 05:17 PM | Print | E-mail | Track Comments

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In previous articles, we've used the 555 test circuit as a hot liquid level indicator for the vision impaired and a cable tester as examples of some of the things that can be used as inputs to the 555 test circuit. In this article, we’ll take a look at one of the output signals that can be generated by the 555 test circuit.

In the above designs the output pin (pin 3) is connected to ground (-). The tone generated with pin 3 to ground is generally useful for qualitative analysis—that is, when a precise measurement isn’t necessary. In the case of the hot liquid level indicator, a precise measurement wasn’t needed, simply an alarm when the hot liquid reached the snaps inside the cup.

If you look at the diagram below you will notice that the pulses are very brief. This means that the .02uf capacitor is charging and discharging very quickly.

In the first oscilloscope screen shot (beneath the circuit diagram) the resistance on the potentiometer is set very low and the pulses are output to the speaker very quickly making a high pitched tone. In the last oscilloscope screen shot at the bottom, the resistance is set very high thus the pulses output to the speaker are much farther apart and the tone is much lower in pitch.

It would seem then that I could take advantage of these pulses and set the resistance to generate a particular tone. If for, example, I replace the .02uf capacitor with a .1uf capacitor I could bring the pitch of the tone to within range of my guitar. In the diagram below on the oscilloscope screenshot you’ll see the pulses are output to the speaker quickly enough to generate a tone that is 246.06 Hz.

This is a computer simulation and I was unable to adjust it to an exact 246.9 Hz, but in reality I can adjust the potentiometer to 246.9 Hz which is B3 or the B string on my guitar.

Here’s the Snap Circuits Designer Diagram if you want to build the 555 Guituner using Snap Circuits (a simple guitar tuner circuit I call the “555 guituner,” see this article for more information regarding the guitar tuner circuit and see this article for more information on how the "guituner" was improved ):

Parts needed:

555 Timer IC (I used a KIA555p, but the NE555 will do just fine)

Snap Circuits Parts:

1Base Grid (11” x 7.7”) # 6SC BG

1 Eight-Pin IC Socket # 6SC ?U8

1 0.1uF Capacitor # 6SC C7

1 Variable Resistor #6SC RV

1 Whistle Chip # 6SC WC

1 4.5 Volt Battery Holder # 6SC B3

1 Slide Switch # 6SC S1

1 Single Snap Conductor # 6SC 01

7 Conductor with 2-snaps # 6SC 02

5 Conductor with 3-snaps # 6SC 03

1 Conductor with 4-snaps # 6SC 04

1 Conductor with 5-snaps # 6SC 05

2 Conductor with 6-snaps # 6SC 05

The frequency of the pulses can be manipulated for any number of uses such as an alarm for hot liquids, tuning a guitar, or what have you. Let’s say that you set the frequency to 10 Hz or ten pulses per second.

What you could do is set up a circuit that could count the pulses. Refer to the picture below and you’ll see a count of pulses from one to three.

Where there is only one pulse your circuit could count to 1. Where there are two pulses the circuit counts to 2. Three pulses would equal 3, and so on. This sequence of pulses is sometimes called a pulse wave or pulse train. The term “pulse train” might be vaguely familiar to you, but if you can’t quite remember where you've heard the term before, allow me to give you a hint:

So, the pulse train on a rotary phone was known as “pulse dialing.” According to Wikipedia:

“On the rotary dial, the digits are arranged in a circular layout so that a finger wheel may be rotated with one finger from the position of each digit to a fixed stop position (finger stop). When released at the finger stop, the wheel returns to its home position by spring action at a speed regulated by a governor device. During this return rotation, the dial interrupts the direct electrical current of the telephone line (local loop) a specific number of times for each digit and thereby generates electrical pulses that the telephone exchange decodes into each dialed digit. Each of the ten digits is encoded in sequences of up to ten pulses.” Source

If you inserted your finger in the finger wheel at the position marked “1” and rotated the dial to the finger stop and released the wheel, you would hear one click in the receiver, or handset as the wheel rotated back to its original position. If you dialed “2” you would hear 2 clicks, if you dialed 3, you would hear 3 clicks, and if you dialed “0” you would hear ten clicks in the receiver. Below is a video demonstration of the rotary dialing device. From the Wikipedia page: “In the video, the green LED shows the dial impulse pulses and the red LED shows the Dial Off Normal contact function. That is, the green LED flashes the count of the number dialed. Also the finger wheel is under the device and not shown in the video but you can hear the demonstrator rotating the finger wheel.

Let’s assume a tone of A2, or 110Hz (actually the frequency doesn't matter in this case since a precise measurement isn't needed so, I chose A2 arbitrarily). The pulses might look something like this:

It might then be possible to switch the circuit on and off and create a coding system. In the next example we switch the circuit on for four cycles, then off for two cycles, on for four cycles, off for two cycles, on for four cycles, off for two cycles, then on for eight cycles, and so on:

If the circuit were off, that could represent a “0” and if the circuit were on, that could represent a “1.” This would be an example of binary language, or the language of computers.

By switching the circuit on and off at periodic intervals we generate a sequence of ones and zeroes, “10101011011011” and if you’re an amateur radio operator, pilot, air traffic controller, first responder, or have been in the military, you might recognize this particular sequence of ones and zeroes as:

The three “dots” represent the letter “S” and the three “dashes” represent the letter “O” in Morse code and in the following video I demonstrate “SOS” the international distress signal in Morse code:

Here’s the build for the Morse code circuit:

Parts needed:

555 Timer IC (I used a KIA555p, but the NE555 will do just fine)

Snap Circuits Parts:

1Base Grid (11” x 7.7”) # 6SC BG

1 Eight-Pin IC Socket # 6SC ?U8

1 0.02uF Capacitor # 6SC C7

1 Variable Resistor #6SC RV

1 Whistle Chip # 6SC WC

1 4.5 Volt Battery Holder # 6SC B3

1 Slide Switch # 6SC S1

1 Press Switch # 6SC S2

1 Single Snap Conductor # 6SC 01

6 Conductor with 2-snaps # 6SC 02

5 Conductor with 3-snaps # 6SC 03

1 Conductor with 4-snaps # 6SC 04

1 Conductor with 5-snaps # 6SC 05

2 Conductor with 6-snaps # 6SC 05

The Morse code example is demonstration of On-Off Keying for the amateur radio operators out there, but it is also called on/off modulation which is a simplified method of digital modulation, or  Analog to Digital Conversion.

NOTES: