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I am trying to make a simple flashing LED toy to get a 5-year-old boy interested in electronics.

  • The LED should blink for the shortest time possible
  • Remain off for several seconds between flashes
  • The battery should last as long as possible
  • No transistors or integrated circuits

This post offers several circuits, but they all require transistors or ICs.* Surely, there is a way to make an LED flash intermittently without any ICs or transistors? Besides the compulsory timing resistor and capacitor, diodes are fine.

I made a novelty flashing light years ago, which used an NE-2 neon lamp, a capacitor, and a resistor. Powered by a couple of unobtanium 67.5 volt batteries, it would flash every ten seconds or so. It did that for many years, until the batteries were exhausted. When the capacitor across the lamp reached the lamp's ionizing voltage, the capacitor discharged until the next cycle.

Now, while LEDs certainly have different characteristics than a gas lamp, there should still be a way to make it flash like the neon lamp did using only passive devices (with the exception of some diodes). How can this be accomplished?


Are there any suitable tunnel diodes for this?

I couldn't find much information in a Google search.


LED voltage curve From this webpage.

In my experience, LEDs glow with just a few mA.


* Only later can we introduce him to active devices. (Long story.)

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    \$\begingroup\$ "The LED should blink for the shortest time possible." You won't be able to see that. Specify a time range in ms instead. You need something that won't conduct until your capacitor has charged up and then will conduct until the capacitor has discharged. Have a look at the characteristics of a DIAC. \$\endgroup\$ – Transistor Jan 21 at 21:50
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    \$\begingroup\$ @Transistor, as long as it produces enough photons to be seen (which depends on background light), you'll see it regardless of how short the pulse is. Now, if you have an LED that's normally on and turns off for only a few ms, that you won't see. \$\endgroup\$ – The Photon Jan 21 at 22:00
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    \$\begingroup\$ you can buy an LED that flashes, without external components, when voltage is applied \$\endgroup\$ – jsotola Jan 21 at 23:51
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    \$\begingroup\$ The "related answer" that you linked uses transistors. You specifically stated "without any...transistors". You give us nonsense requirements..."as short as possible" and "as long as possible". It sounds like you already have a solution in mind, and you just keep telling us how wrong we are that we don't see it. \$\endgroup\$ – Elliot Alderson Jan 22 at 1:28
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    \$\begingroup\$ @Mike Waters, your idea "to get a 5-year-old boy interested in electronics" in such a way is just wonderful. I remember, it was in 60's, when I (about 10-year old boy) was playing with a DC bell... and I asked myself, "How does it vibrate"? Much later, I became acquainted with the Don Lancaster's "elegant simplicity" - your neon lamp arrangement. What you need to build such a relaxation oscillator, is an element with S-shaped negative resistance... having a hysteresis... \$\endgroup\$ – Circuit fantasist Feb 2 at 22:47
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You need a wheel. A bicycle wheel should work, and the flashing light makes sense on a bike. Some other wheel may work, depending on what do you have at hand. Maybe a fan would work.

Buy a magnet and reed contact like these. Connect battery, LED, resistor and the reed contact in place of the switch. Now just approach the magnet to the reed contact, it should trigger, lighting the LED.

schematic

simulate this circuit – Schematic created using CircuitLab

Attach the magnet to the wheel and place the reed contact nearby. As long as the wheel is spinning (by hand, or by motor if it is a fan, or bike is moving) the LED should flash when the magnet is passing near the reed.

As @Wossname commented, a pendulum would also work instead of a wheel. You can use an old pendulum clock without any electric components.

To satisfy all requirements of the question "in depth", you would need a small electric engine rotating the wheel, probably with reductor. Or a steam engine if you really dislike the electricity, up to you. However the mechanical complexity may not outweight the lack of one another transistor.

BTW the reed, the LED and the resistor all have nice, long pins, easy to wire, and the child may be happy to play just holding the magnet in his hand.

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    \$\begingroup\$ Or perhaps a pendulum instead of a wheel. This would also make a nice introduction to simple harmonic motion. \$\endgroup\$ – user98663 Feb 3 at 21:07
  • \$\begingroup\$ A cluster of inventive ideas... \$\endgroup\$ – Circuit fantasist Feb 3 at 21:17
  • \$\begingroup\$ Here's another fun story about "invention"... I remember, as a ten year old boy, I “invented” something like an inertia sensor or what at present, in security systems, is referred to as "tilt sensor". I fixed the screw cap of a small electric bulb to an elastic terminal of a 4.5V flashlight battery so that the other bulb lead should touch lightly the stiff battery terminal. Then I enclosed this “device” in a box and launched it in a basin full of water. The “ship” slowly swung and the bulb rhythmically flickered, imagine, by itself! ! My family was enraptured with my spirit of invention. \$\endgroup\$ – Circuit fantasist Feb 3 at 21:26
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Flashing requires some kind of active device. Either a semiconductor like a transistor, or a electromechanical one like a relay, or something exotic like high voltage discharge devices.

Here is how you can make a LED flash using a relay and a capacitor: https://www.youtube.com/watch?v=T_az4omJx0k

This is how the turn signals on cars often works.

But if you want low power consumption, then a transistor solution is the way to go.

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  • \$\begingroup\$ "Flashing requires some kind of active device. Either a semiconductor like a transistor, or a electromechanical one like a relay, or something exotic like high voltage discharge devices." Dare I say that it may be possible using the led. As an active component \$\endgroup\$ – tuskiomi Feb 3 at 15:31
  • \$\begingroup\$ Turn signal in old cars works with thermal relay that latches when cold and heats itself when latched. This design has no capacitor at all. \$\endgroup\$ – h22 Feb 3 at 19:09
  • \$\begingroup\$ The relay circuit exploits the fact that relays possess hysteresis. As far as I can remember, 12 V auto relay can switch on at about 9 V and off at 6 V. \$\endgroup\$ – Circuit fantasist Feb 3 at 22:41
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The LED should blink for the shortest time possible... Surely, there is a way to make an LED flash intermittently without any ICs or transistors?

You don't need anything. White LEDs have a phosphor which converts blue light to 'white'. However the phosphor also acts as a scintillator which produces pulses of light in response to natural background radiation. Only one photon at a time and so intermittent that it will be too weak to see - but this is what you want!

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  • \$\begingroup\$ Definitely a very original answer... \$\endgroup\$ – Circuit fantasist Feb 3 at 21:08
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Why not use a neon?

I made a novelty flashing light years ago, which used an NE-2 neon lamp, a capacitor, and a resistor. Powered by a couple of unobtanium 67.5 volt batteries, it would flash every ten seconds or so. It did that for many years, until the batteries were exhausted. When the capacitor across the lamp reached the lamp's ionizing voltage, the capacitor discharged until the next cycle.

Perhaps you're speaking of the neon circuit presented in "Neon Novelty", by B.C. Macdonald, Everyday Electronics, vol 2 no 3, March 1973 (p60 of PDF).

I recently (2016) made one of these with eight neons. In place of the "unobtainium" 90V valve radio battery (eg B126) ... I used ten PP3 9V batteries. Works perfectly!

In 1973 I got a good shock off the B126 battery. I'm sure it's not advisable to let a 5-year-old child loose with a 90V circuit, but how you build it will make it safe or dangerous.

enter image description here Everyday Electronics, vol 2 no 3, March 1973, p140.

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  • \$\begingroup\$ This idea is older; it is proposed by Pearson & Anson in 1922 - en.m.wikipedia.org/wiki/Pearson%E2%80%93Anson_effect \$\endgroup\$ – Circuit fantasist Feb 4 at 7:15
  • \$\begingroup\$ @Circuitfantasist it was the questioner's phrase "novelty circuit" which is what reminded me of this particular article; of course a hobbyist magazine wouldn't be the original source. Thanks for the proper name though! \$\endgroup\$ – jonathanjo Feb 4 at 9:01
  • \$\begingroup\$ I read the article with great pleasure... and warmth invaded my soul... How well it is described... with passion and enthusiasm... Only the main neon lamp property (hysteresis) is not emphasized - that after the lamp is lit it "memorizes" this state until the voltage across it drops to the lower threshold. I'm trying to imagine how the full multistage circuit works... a big challenge to my imagination... \$\endgroup\$ – Circuit fantasist Feb 4 at 10:36
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I will try to summarize these specific circuit solutions into a "philosophy" of the simplest possible relaxation oscillator.

In such an arrangement, some substance like water, air, sand, data, money, etc. accumulates in a tank and its level is constantly rising (it is moving in one direction). In our case, this is electric charge (potential energy) in a capacitor. It is charged by a voltage source (through a conductive path in series) so its voltage "moves" towards the positive supply rail. Finally, it approaches the rail and stops there. The problem is, "What do we do to keep this movement going forever?"

The solution is to reverse the movement direction (just like we swim back and forth in a swimming pool) by discharging the capacitor. We can do it by connecting another conductive path in parallel to the capacitor (even without disconnecting the charging path). The voltage will begin "moving" towards the negative supply rail. When it approaches the rail, we reverse its "movement" by charging the capacitor again...and so on so forth...

To automate this arrangement, we need a switch with memory that is toggled when the voltage reaches the supply rails (in a manner the end switches control a motorized curtain). It can be implemented in a few ways. Some of them are more sophisticated as in timer 555 built by threshold comparators, latch and transistor switch. Other are simple, e.g. consisting of a 7414 Schmitt trigger and RC circuit. But the "elegant simplicity" is to connect a 2-terminal element combining memory and switching functions in one. What is this mysterious "2 in 1" element?

It behaves as an "overacting dynamic resistor" which resistance, in some regions, significantly depends on the voltage across it. Initially, at low voltage, it has relatively high resistance. The capacitor charges and the voltage across it increases. Then, at some voltage level, the resistance sharply decreases in an avalanche-like manner... and stays in this state until the capacitor discharges and the voltage reaches the low voltage threshold. Then, the resistance sharply increases and the capacitor begins charging again...

This element is known as a negative differential resistor with S-shaped IV curve. When driven by voltage, it has such a behavior of a Schmitt trigger. Simply speaking, it is a dynamic resistor with memory (aka hysteresis). The neon lamp is an example of such an element with S-shaped curve.


Maybe, it would be interesting for you to understand how this magic element "jumps" when switching (it is not well explained in sources). Look at the two pictures below. To show in detail the mechanism of operation, two separate graphs are presented. The first is for the case when the voltage across the element increases; the second is when it decreases (elements with hysteresis has different behavior depending on the direction of the input change). When superimposed, the two partial curves compose the whole hysteresis curve.

In this mode, there is in total three intersection points of the two superimposed IV curves: the middle point is unstable; only the end points are stable. The IV characteristic is a multivalued function and the output quantity can take only the end stable values. The switching between the two states is an avalanche-like process accelerated by the intrinsic positive feedback. Beginning from the one end value and "looking for" the equilibrium state, the negative resistor changes vigorously but in the "wrong" direction its instant resistance. Thus it recedes further and further from the equilibrium point in an avalanche-like manner and finally reaches the other end value.

Increasing voltage (Fig. 1). Look at the IV curve (blue) of an S-shaped NDR driven by a voltage source (red). When increasing voltage reaches VH, the instant resistance decreases momentarily. Its IV curve (orange) rotates counterclockwise; the operating point A moves up ("jumps up") along the voltage source IV curve and pictures this vertical part of the curve. Thus during the jump, the current increases instantly (jumps up) but the voltage stays constant.

Bistable_decreasing_voltage

Fig. 1. S-shaped NDR driven by increasing voltage

Decreasing voltage (Fig. 2). When decreasing voltage reaches VL, the instant resistance increases momentarily. Its IV curve rotates clockwise; the operating point A moves down along the voltage source IV curve and pictures this vertical part of the curve. During the jump, the current decreases instantly (jumps down) but the voltage stays constant.

Bistable_decreasing_voltage

Fig. 2. S-shaped NDR driven by decreasing voltage

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If you're asking what I think you're asking for, you need a simple timing circuit. The simplest timing circuit that I've come across so far is a relaxation oscillator. For example:

Relaxation Oscillator Built with a Transistor

Image source: Learning about Electronics - How to Build a Relaxation Oscillator with a Transistor

This, however, is not very reliable, and I've found it only works on a sunny day. You can however create a relaxation oscillator with a comparator:

Op amp-based hysteretic oscillator, with the assumption of same-valued resistors

Image source: A comparator-based hysteretic oscillator from Wikipedia - Relaxation oscillator

If you're looking for an oscillator out of only passive components, I can say it is very much in the realm of possibility. You'll need to spend a lot on high voltage power supplies; 6kV should do. The circuit is commonly used in gas lasers, and if proper safety precautions are taken, can be used in atmospheric air (though precautions should be taken, as UV radiation is emitted.)

Schematic diagram of pulsed copper vapor laser with basic power supply

Image source: Fig. 24, Schematic diagram of pulsed copper vapor laser with basic power supply from Module 4-2, Gas Laser Power Supplies

Just remove the thyratron, and tune the RLC accordingly. You can read more here.

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  • \$\begingroup\$ These contain ICs and transistors however. \$\endgroup\$ – h22 Feb 3 at 8:13
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    \$\begingroup\$ @h22 that's intentional. Ever see an answer to a question that said ”don't do that"? That's this. \$\endgroup\$ – tuskiomi Feb 3 at 8:14
  • \$\begingroup\$ The IV (output) curve of the reverse connected transistor above should be S-shaped. It is interesting to see it. For this purpose, it should be driven by a current source. I will try to measure it with a simple IV curve tracer in the lab. \$\endgroup\$ – Circuit fantasist Feb 4 at 8:47

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