Take a clear box and wire it up using some colored christmas lights, the kinds that work on 110 VAC. Take two SPST switches, big enough for you to disassemble and insert SMD diodes into. One switch has the diode one way and the second switch has it the other way. I'll post a schematic shortly. Wire a single series wire from the point where it enters the clear box, going to the first of two light sockets. From the first light socket to the second, run an additional wire. From there, wire the second light socket to one of the two switches. Then on to the next. Then back out of the box. What it ends up looking like is two lights in series with two series switches. No light should light up unless both switches are on. And when both switches are on both lights should light up.

Finally, carefully work the base of the lights off of the bulbs. Inside the base, wire up two more diodes, one one way and the second the opposite way. Use two different colored lights for obvious reasons (obvious to come).

Each bulb works on 110 VAC. But with the diode in one only the current will flow when flowing against the diode. Each switch having a diode in them will also prevent the current from flowing in both directions. Let's assume you have a red and a green bulb and one switch with a red handle and the other with a green handle. Plug the rig in and turn on both switches. Immediately both lights come on. Turn off the red switch and the red bulb (if you wired it correctly) will go off but the green will stay on. Turn the green switch off and the red on and the green bulb goes out and the red bulb comes on.

Now: To PROVE you didn't hide any wires, swap the bulbs in the bases. Put the red bulb in the place of the green bulb and the green in the red. Again, actuate the switches. Even though you've swapped positions, when you turn the red switch is turned on the red bulb lights. Regardless of which base it's screwed into. The secret is all in the diodes. As promised, a schematic will be forthcoming. But those who look at the circuit will be perplexed to figure out how the red switch controls the red bulb regardless of which socket it's screwed into.

 

Here's the schematic and how it works:

When S1 is closed current flows CW through D2 & D3. D4 blocks current and forces it to go through the red filaments. When S2 is closed current flows CCW through D1 & D4. D3 blocks current and forces it to go through the green filament. Regardless of which position the lamp is in only the green lamp will light with the green switch. Same is true of the red. It's important to hide the diodes inside the switches and inside the bases of the lamps.

 

Simple computer trick. No software or electronics.

Cut a square corner from a Post-It note with the adhesive on it. When the victim is away from his computer, turn his mouse over and cover the led with the piece of the Post-It note. Walk away calmly.

With practice, you can do this without being noticed. When the victim returns, watch the hilarity ensue

 

square corner from a Post-It note

Done that plenty of times. Ever take transparent tape (Scotch tape) and make a short loop with the sticky side out? Stick it to the telephone ear piece. Watch the reactions when someone answers the phone.

Had a supervisor who always took my pens. I took one apart (Bic) and removed the guts. Filled the hollow tube with silica sand and put the cap back on. Jammed it on tightly. She had a real hard time getting the cap off. When she did she spread the sand all over her desk. Immediately she hollered my name. I acted ignorant to the whole thing. Finally she laughed and said "OK, you got me."

Another: Had a GIF called "Jurassic Fart" Click here for video. Took the sound track and isolated the pig fart and uploaded it to a colleague's computer then set his start chime to play that fart. He was notorious for leaving his speakers turned up loud. The following morning when he turned on his computer and it got to the part where, as I call it, the angles sing, there was that deafening fart, which made EVERYONE turn and exclaim his name!

 

Done that plenty of times. Ever take transparent tape (Scotch tape) and make a short loop with the sticky side out? Stick it to the telephone ear piece. Watch the reactions when someone answers the phone.

Had a supervisor who always took my pens. I took one apart (Bic) and removed the guts. Filled the hollow tube with silica sand and put the cap back on. Jammed it on tightly. She had a real hard time getting the cap off. When she did she spread the sand all over her desk. Immediately she hollered my name. I acted ignorant to the whole thing. Finally she laughed and said "OK, you got me."

Another: Had a GIF called "Jurassic Fart" Click here for video. Took the sound track and isolated the pig fart and uploaded it to a colleague's computer then set his start chime to play that fart. He was notorious for leaving his speakers turned up loud. The following morning when he turned on his computer and it got to the part where, as I call it, the angles sing, there was that deafening fart, which made EVERYONE turn and exclaim his name!

It isn't Aprils fools without using a camera flash PCB to charge a SMPSU reservoir cap and leaving it on the bar in the pub...…….

 

Have to admit, I've charged a few caps and left them laying around too.

 

It isn't Aprils fools without using a camera flash PCB to charge a SMPSU reservoir cap and leaving it on the bar in the pub...…….

That so reminds me of a trick we played on another student in school. EF used to come into the lab, turn on his overhead desk lamp (the old fashioned hanging lamps), then leave forever. We guessed he wanted the professor to think he was working. He took a very long time to finish his degree. Anyway, we (the other students in the group) replaced that bulb with a 110 V flash bulb. Next evening, lights were out, and true to form, EF came in to turn on his light.

Of course, we were huddled in an adjoining lab just cracking up. EF showed up at the door with only two words, "Very funny!" and huffed away.

 

Have to admit, I've charged a few caps and left them laying around too.

Megger insulation testers were fairly plentiful on the 1st year EE course...………….

 

Used to use ignition coils and a couple of D cells...

 

It was not for April 1, but I did do an interesting trick, after I found that some were messing with my computer at work. It switched on and off at a lug strip, computer, monitor, and memory accessory. I added another plug that powered a time delay relay that switched on after 20 seconds, and activated a second relay wired with a self-sealing contact . That relay was hidden, plugged into the mains and powering a hidden 12 volt supply that fed an auto alarm siren and a fire alarm bell, all out of sight under my desk. So after 20 seconds the noise would begin and there was no obvious way to switch it off. That required finding the second plug that was not easy to see. The next day I came in and a bunch of folks looked at me funny and nobody ever messed with my computer again. All because the management did not do anything about messing with my computer. We each had our own, mine was not one of the better ones.

 

Used to use ignition coils and a couple of D cells...

How did you stop the 25kV (ish) capacitor charge dumping back into the HT winding - I imagine hooking up an EY51 + heater supply would've been a bit cumbersome...…..

 

How did you stop the 25kV (ish) capacitor charge dumping back into the HT winding - I imagine hooking up an EY51 + heater supply would've been a bit cumbersome...…..

We didn’t know about all that in the 4th grade. All we were aware of is that it was quite of jolt. Also, I think the part was just a coil. Think 1950-1960 autos.

 

Got my hands on a VERY old relay. About 3 inches tall with the armature on top and the contacts on the side. I noticed that it would hold the contacts closed for a moment after the power was cut. So I wired it to turn itself on when the relay contacts were in the NC position, thus interrupting its own power. Going on memory, it seemed to operate about 3 clicks per second, that's how slow it was in releasing. I then wired the NO contacts to an automotive ignition, a bunch of roofing nails and a board. Connected one (ground) wire to one end of the nails and the HV output to the other end. The heads of the nails were very close together, but there was a definite spark gap between them. I think I used about 8 inches of space from the first to the last nail head. And yes, I could throw the spark all the way across every last nail and gap. Today I would argue that such a thing can not be done. There's a limit to the amount of air gap the spark can travel. But I did imagine this (at the time) as being a great way to dissuade people from putting their hands on top of the fence rail. I'm sure the spark couldn't have gone that far.

 

Got my hands on a VERY old relay. About 3 inches tall with the armature on top and the contacts on the side. I noticed that it would hold the contacts closed for a moment after the power was cut. So I wired it to turn itself on when the relay contacts were in the NC position, thus interrupting its own power. Going on memory, it seemed to operate about 3 clicks per second, that's how slow it was in releasing. I then wired the NO contacts to an automotive ignition, a bunch of roofing nails and a board. Connected one (ground) wire to one end of the nails and the HV output to the other end. The heads of the nails were very close together, but there was a definite spark gap between them. I think I used about 8 inches of space from the first to the last nail head. And yes, I could throw the spark all the way across every last nail and gap. Today I would argue that such a thing can not be done. There's a limit to the amount of air gap the spark can travel. But I did imagine this (at the time) as being a great way to dissuade people from putting their hands on top of the fence rail. I'm sure the spark couldn't have gone that far.

I did it with a 555 and MOSFET TAC when chavs kept meddling with the plug caps on my motorcycle - the extra coil had a damaged plug cap on the end of the HT lead.

 

I also took a project box, a relay, a piezo buzzer, a couple of microswitches with a leaf and batteries.

I drilled two holes in the project box.

One wide enough for the leaf. I bent the leaf so it fit through the larger hole and mounted the first microswitch so it’s bent leaf protruded through the hole. I then wired the NO contact to the relay.

The relay was wired to latch itself. The same lead went to the piezo buzzer.

However, in series with the latching wire was the second microswitch’s NC contacts. It was mounted in line with the small hole.

The battery + leaf went to the first microswitch. Ground was connected to the relay and buzzer.

I left it on my desk. Inevitably, someone would pick it up. The buzzer blasted. There was no obvious way to shut it off.

That’s where the tiny hole came into play. A bent paper clip pressed the second microswitch, interrupting the latch current.

I embellished it over the years, hiding it in a candy bowl, labeling it DANGER or Do Not Touch. It was always good for a laugh.

 

Do Not Touch

I like this one. Can you post a schematic?

 

I like this one. Can you post a schematic?

Sure! After the weekend.

 

I also took a project box, a relay, a piezo buzzer, a couple of microswitches with a leaf and batteries.

I built something like this except all it did was light an LED.

The owner of the business found it and picked it up. It lit.

He tracked me down and said, “OK, what have I done?”. He was sure something bad was happening.

 

Simple computer trick. No software or electronics.

Cut a square corner from a Post-It note with the adhesive on it. When the victim is away from his computer, turn his mouse over and cover the led with the piece of the Post-It note. Walk away calmly.

With practice, you can do this without being noticed. When the victim returns, watch the hilarity ensue

Don't know about most people, but anytime my mouse doesn't work, the first think I do is move it briskly back and forth a couple times and if I still see no response then the second thing I do is look at the bottom of the mouse to see if the LED is actually on. That seems pretty natural to me, but then I know how optical mice work. Perhaps I just assume that most people know enough to at least know that the bright red light on the bottom is important.

But I can try it with my daughter and see how she reacts.

 

One I've heard about that I like is the Hidden Cricket.
It's a small battery powered circuit that makes a cricket chirp sound when in the dark, but it has a light sensor that turns it off.
So you place it in some dark location where it periodically chirps, but as soon as someone turns on a light to find it, it stops, making it a devil to find.