Birthday badges teach kids how to solder

[Ian Lee, Sr] wanted to have an educational activity at his younger son’s birthday party. These were uncharted waters for him as he doesn’t remember education taking place at his own early birthday parties. But he came up with a great idea, with was to teach soldering using interactive badges which each guest could assemble themselves. He needed about twenty, so he tried to keep the BOM as small as possible. But that didn’t mean skimping on features.

You can see the black LED-type package on the left of the assembled badge above. This is an IR receiver whose counterpart transmitter is on the right side of the board. When two of these get within 6-8″ of each other the start talking back and forth. There is no microcontroller involved, instead the system relies on a multivibrator design. One of the red LEDs at the corner of the ‘smile’ is always blinking. When it is off, the IR transmitter is powered. This is picked up by another badge’s receiver, which lights the second ‘smile’ LED. You can see this happen in the short clip after the break.

Although there are relatively few components that went into this, it would take the kids a long time to put them together as they’re just learning. [Ian] and his eldest son soldered on all of the components except for the resistors beforehand.

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Decades old TV gets sleep function thanks to 7400 family of chips

[Viktor] dredged up a hack he pulled off years ago. His grandfather likes to end the day in front of the TV, but he falls asleep soon after sitting down. Rather than tick away the electricity meter all night, [Viktor] built an automatic shutoff which is akin to a modern TV’s sleep feature.

At the time microcontrollers were not as easy to source as they are now. So [Viktor] used a circuit based on the 7400 family of logic chips. It uses a multivibrator to feed some binary counter chips. These are used to divide the oscillations to establish the desired timing. He tuned the system to be about 15 minutes, but that can be adjusted using a potentiometer built into the multivibrator. When time is about the run out an LED next to the TV comes on. This way if [Viktor's] grandfather is still awake he can press a button next to his chair to reset the counter. But if he’s already snoozing the counter will eventually switch off the television.

Multivibrator in theory and practice

We must admit that we’ve been guilty of using a microcontroller to make two LEDs blink alternately in the past. It’s not the worst transgression, but it stems from our discomfort with analog circuits. Luckily, [Ray] published an illustrated guide on building multivibrator circuits. This is a simple method of assembling a two-output oscillator. All it takes is a pair of NPN transistors, which are then switched by on and off based on a resistor-capacitor (RC) timer.

[Ray] does a good job of walking us through how the circuit works at each stage of one complete cycle. You’ll need to read carefully, but the supplementary schematics he uses to water down snap shots of the various electrical states really helped us understand.

Of course, blinking LEDs isn’t the sole purpose of a multivibrator. It is a method of producing a clean square wave which can be used as a clock signal for TTL logic chips.¬†Oh, who are we kidding, see the blinky goodness for yourself in the video after the break.

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Pure TTL based clock

We’ll just say, [Kenneth] really likes clocks. His most recent is a pure 7400 series TTL based one, ie no microcontroller as seen in the past, here, here, and here. The signal starts out as a typical 32,768 crystal divided down to the necessary 1Hz, which is then divided again appropriately to provide hours and minutes.

As far as TTL clocks go, this is nothing too original; until it comes to his creative button interface. By using a not as sexy as it sounds multivibrator, he can produce a clean square wave instead of the figity signals produced from buttons to advance and set the time. Like always, he also provides us with a thorough breakdown of his clock, after the jump. Continue reading “Pure TTL based clock”