Is there a way to make an LED blink using a passive circuit?

Question

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?

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Are there any suitable tunnel diodes for this?

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

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From this webpage.

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

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

Answer 1

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.

Answer 2

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.

^{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.

Answer 3

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:

_{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:

_{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.)

_{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.

Answer 4

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!

Answer 5

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.

Everyday Electronics, vol 2 no 3, March 1973, p140.

Answer 6

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.

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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.

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.

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

Answer 7

LEDs with an internal flashing circuit are available widely, you will need an external resistor, the battery & that's all. a AL-B513ID-004 flashes at about 2.4 Hz You won't even need the resistor if the battery is less than 12v.