Electronic – How to protect an electronic construction kit (a toy) from damages

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As I child, I had this electronic construction kit called Elektronik 1. It was manufactured in former Czechoslovakia, based on resistors, diodes, BJT transistors, capacitors, an LW+SW radio antenna and an earphone. It is quite simple, very good for children like 8 years old. It was originally powered from a 4.5 V battery (3 x 1.5V).

Elektronik 1

Now I try to recover the kit. I realized that diodes are all dead, some transistors are dead; all the other parts seems to be okay (including all those 30 years old capacitors). The original parts from eastern Europe aren't manufactured anymore, but it was easy to find some similar replacements. But the main question now is: Why those parts were damaged and how to protect them from another damage? When children play with this stuff, they can do many stupid circuits, because they are simply curious what will it do. So please tell me if you have got some tips what can be done there to protect those semiconductors from damages. Or at least a little bit.

The problematic parts are:

BJT transistors, I use BC546A and BC556A now.
Rectifier diode, I use 1N5817 now – it has 1 A limit, hopefully it helps.
Signal sensing diode, I use 1N5711 – this one is very sensitive.

Besides the semiconductor parts, I also want to use AC/DC adapter instead of quite expensive 3R12 type 4.5V battery. Of course, I don't know current limit of the original kit – how many amps do I need to replace a battery? I think 0.2 A could be enough. I now use an old Nokia charger. It is rated 3.6V 0.33A 1.3VA, but it is actually unregulated and gives up to 10 volt without load with audible 50 Hz noise when used for an LF amplifier. I think I will add a large capacitor and LM317LZ to regulate it down to 4.5 volt. I hope that LM317LZ will also act as a safe current limiting device which can possibly also protect the semiconductors from future damages.

Best Answer

If you really want to make this 'kid proof' (I have more experience with 'student proof', that might be a slightly weaker test) you should limit the voltage AND CURRENT delivered by the power source, and select each and every component to withstand that current and that (reverse!) voltage.

100 mA @ 4.5V is probably enough. Use two LM317s (one for current limit, one for voltage limit) or even a vintage ua723 for the power.

That sensing diode might be a problem. Would the 1N5817 not do? Or maybe you can put a small resistor in series with that 1N5711 to limit the short-circuit current to somthing it can handle.

For the transistors I would take something more beefy, maybe BD139/140.

Related Solutions

Electronic – / how can I get a student-safe power supply for electronics projects

I know you said you don't want to run outlets everywhere, but I think a pile of wall warts is exactly what you want. Jameco has a large selection. 9 V sounds pretty good and that is one of the standard voltages, but you might want to consider 5 V (see below). Get something in the 500 mA to 1 A range.

These supplies are inherently isolated from the line, are usually short-circuit protected (check to make sure, you definitely want that), and draw so little AC power that you can string a bunch of outlet strips together without harm.

In the end, I think this will cost less and provide a better experience for the kids. I remember when I was a kid tinkering with this stuff how frustrating it was to have batteries run down, especially when you're not aware of when you are asking a lot from them. You also feel a lot less guilty abusing a power supply than running down consumable batteries.

You can start a fire with almost anything. If you do just the right thing, even a 9 V 500 mA supply can catch something on fire, but no more so than a 9 V battery and without the chance of the battery itself doing something bad and causing chemical burns.

If you are worried about LEDs getting damaged by them getting hooked up backwards, maybe you should get 5 V or 6 V supplies. Most LEDs can handle 5 V in reverse, and some 6 V. With 5 V supplies, you can eventually run logic cicuits directly without having to use a linear regulator. Lots of stuff will run well from 5 V. If you get 5 V supplies, get at least 1 A capability. That will be useful for running small motors. 5 W total power really isn't all that much.

Electronic – Electronic Temperature Control Sound Light Alarm DIY Kit – how does it work

The kit's schematic is wrong, and and if it's wired up to follow the schematic, the PCB will be bad.

R2 needs to go between the collector of Q1 and the base of Q2.
The wire going from R2 to the collector of Q3 needs to be eliminated.
D1 and the beeper need to be connected in parallel, with the + side of the beeper connected to 3 volts and the otherr side connected to Q3 collector.
Check the current through the LED. If it's more than specified, you'll need to add some resistance in series with the LED.

Here's the schematic, and here's how it works:

RT1 is an NTC thermistor, and in conjunction with RV1, (a 10k pot wired as a rheostat) comprise a voltage divider controlling the voltage on Q1's base.

As the ambient temperature rises, RT1's resistance falls, eventually getting the base of Q1 close enough to +3 volts to start turning it off.

When that happens, the current through R1 will start falling, reducing the voltage/current available to drive Q2's base.

That'll cause Q2 to turn off, which will cause it to stop shunting the current through R3 to ground and allow it, instead, to flow through Q3's base-emitter junction which will force Q3 into saturation.

When that happens, the LED and the beeper will be, essentially, connected across the 3 volt supply, which will turn them both fully ON.

And, finally, here's the LTspice circuit list with a PMOSFET simulating the NTC thermistor:)

Version 4
SHEET 1 880 680
WIRE -336 -176 -464 -176
WIRE -208 -176 -336 -176
WIRE -16 -176 -208 -176
WIRE 192 -176 -16 -176
WIRE 352 -176 192 -176
WIRE 464 -176 352 -176
WIRE -336 -144 -336 -176
WIRE 464 -96 464 -176
WIRE 352 -80 352 -176
WIRE 192 -64 192 -176
WIRE -208 -48 -208 -176
WIRE -336 -32 -336 -64
WIRE -256 -32 -336 -32
WIRE -336 16 -336 -32
WIRE 352 16 352 -16
WIRE 464 16 464 -16
WIRE 464 16 352 16
WIRE -16 32 -16 -176
WIRE 352 32 352 16
WIRE -464 64 -464 -176
WIRE -208 80 -208 48
WIRE -80 80 -208 80
WIRE 192 80 192 16
WIRE 288 80 192 80
WIRE 192 112 192 80
WIRE -208 128 -208 80
WIRE -336 144 -336 96
WIRE -16 160 -16 128
WIRE 32 160 -16 160
WIRE 128 160 112 160
WIRE -16 192 -16 160
WIRE -464 304 -464 144
WIRE -336 304 -336 224
WIRE -336 304 -464 304
WIRE -208 304 -208 208
WIRE -208 304 -336 304
WIRE -16 304 -16 272
WIRE -16 304 -208 304
WIRE 192 304 192 208
WIRE 192 304 -16 304
WIRE 352 304 352 128
WIRE 352 304 192 304
WIRE -464 368 -464 304
FLAG -464 368 0
SYMBOL pnp -80 128 M180
WINDOW 0 64 66 Left 2
WINDOW 3 41 34 Left 2
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL npn 128 112 R0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -32 176 R0
SYMATTR InstName R1
SYMATTR Value 15k
SYMBOL res 176 -80 R0
WINDOW 0 -41 46 Left 2
WINDOW 3 -49 76 Left 2
SYMATTR InstName R3
SYMATTR Value 6.8k
SYMBOL res 128 144 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 39k
SYMBOL npn 288 32 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL res -224 112 R0
SYMATTR InstName RV1
SYMATTR Value 10k
SYMBOL LED 336 -80 R0
WINDOW 0 -20 -1 Left 2
WINDOW 3 -97 63 Left 2
SYMATTR InstName D1
SYMATTR Value NSCW100
SYMBOL res 448 -112 R0
SYMATTR InstName R7
SYMATTR Value 100
SYMBOL Misc\\battery -464 48 R0
WINDOW 0 12 94 Left 2
WINDOW 3 8 15 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName BT1
SYMATTR Value 3V
SYMBOL res -352 -160 R0
SYMATTR InstName R4
SYMATTR Value 1000
SYMBOL res -352 0 R0
SYMATTR InstName R5
SYMATTR Value 1000
SYMBOL voltage -336 128 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value PULSE(3 0 0 10 10 1U 0 1)
SYMBOL pmos -256 48 M180
SYMATTR InstName Q_Rt1
SYMATTR Value FDR840P
TEXT -338 336 Left 2 !.tran 30 uic

Best Answer

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