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Brainless TritiLED

My attempt for TritiLED without a MCU

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For his always-on coin cell powered LED marker Ted Yapo is using a MCU to generate pulses driving the LED. While use of a MCU gives a lot of flexibility, it should be possible to drive the LED by much simpler circuit with comparable results.

This project is inspired by the TritiLED project. The key idea of the project is that every LED has its optimal current which is typically much higher than a few microamps but much lesser than the rated current of given LED. Its author Ted Yapo uses a PIC MCU and some passives to generate current pulses. He thinks using a MCU for blinking a LED can be considered an overkill and I agree. I would like to make my own version of TritiLED without a MCU. I will rely heavily on Ted Yapo's work and when I do something differently I plan to compare my motives and results with his.

Requirements for the project:

  1. Use common, loosely specced parts
  2. Simple (and cheap) circuit
  3. Predictable, repeatable and reasonably stable parameters
  4. Settable current draw with reasonable minimum 1-2 uA or less

Ad 1.:

Many circuits I find on the web use some components that are not easy to get. It may be some ancient part (such as unijunction transistor), some IC with strict requirements (i.e. very low supply current op amp), or a discrete part with some non-trivial requirements - very low threshold voltage or specific R_DS(ON) of a MOSFET, exceptionally low or high gain of a BJT. This also limits resistor value - 10M is the highest common value of a resistor. Avoiding parts unavailable in THT is also preferred.

Ad 2.:

Paying premium for extending a coin cell battery lifetime by 10% is nonsense. Cost and complexity must be balanced with possible gains. This also limits resistor usage - pair of 10M resistors in series may be acceptable to "create" a 20M resistor. Using 100 of 10M resistors to get 10 of 100M resistors is bad.

Ad 3.:

The parameters (LED current, duty, frequency) should depend mainly on easily measurable parameters - preferably values of passive components. "Nearly constant" voltages as V_BE of a transistor, forward voltage of a diode or a LED may be used too - it is possible to make a guess, measure result and change passives slightly to get the desired result. Characteristics which are highly variable (either part-to-part or temperature dependent) such as transistor gain or diode reverse leakage current should not be used.

Ad 4.:

The quiescent current should be so low that the circuit will keep some reasonable efficiency even at average current drain of 1-2 uA. Since 1 uA is about 10 mAh per year it means run time more than 10 years for one CR2032 cell should be possible.

  • BJT or (MOS)FET?

    Smajdalf07/10/2019 at 15:03 0 comments

      The answer seems easy - everyone knows BJTs are old and power hungry. They may have some marginal applications but FETs rule the world. Yet I believe BJTs may have important advantages over FETs.

      It is true a BJT needs constant base current to conduct while a FET needs only to charge a few pF of Gate capacitance to turn on. If the switching frequency is low (and we aim for very low frequency around 100Hz) FET is seemingly more efficient. But there is a catch - FET needs to be driven by a push-pull output. In discrete components for simplicity it is often realized by a pull-down resistor and an active circuit pulling up (or vice versa). But the power saving advantage is lost here - the pull-down consumes power all the time while the FET is turned on (or off). Very roughly speaking a BJT may be considered a FET with a resistor build in:

      Probably in typical situation using MOSFET still saves some current but the difference may be negligible - and using the BJT may save one resistor.

      There are other advantages in using BJTs:

      1. They are less ESD sensitive than FETs. Modern FETs have often some ESD protection build in but not all of them.
      2. BJTs are "ancient". They are commonly available in both THT and SMD package. Newer FETs are often SMD only. Common FETs available in THT are huge power MOSFETs mostly in bulky TO-220 package with large gate capacitances and high threshold voltages.
      3. BJTs have "unified" base-emitter voltage. It is around 0.6V. Of course it depends on transistor type and base current but in a given circuit replacing a BJT with similar one shouldn't change the V_BE voltage drop by much.

      Especially the point 3. is important for me. It is hard to find a FET that conducts with only 0.6V gate voltage. I do not know any available in THT. Also the region between fully closed and fully conducting is much wider and not well described in datasheets.

      Maybe it is only my lack of understanding but BJTs seem much easier to use than MOSFETs. If a circuit works well with one BJT, there is a good chance it will work with reasonable similar BJT too. This cannot be said for FETs - with the same gate voltage one is closed while another is fully open.

      Since replacing a BJT by a FET is quite straightforward I will use BJTs primarily and switch to a FET only when there is a good reason for doing so.

  • Using an Inductor or not?

    Smajdalf05/29/2019 at 19:32 0 comments

      Ted Yapo uses an inductor to generate the current pulse through the LED. It generates a sawtooth current wave - not optimal to drive the LED but still very efficient according to his simulations. It has two important advantages:

      1. Possibility to use lower supply voltage than the forward drop of the diode.
      2. Increased electrical efficiency

      When using a lithium battery with voltage 3V it should be high enough for any LED. Typical LED is most effective with currents less than its maximum current and it has considerably lower forward voltage than the rated voltage. If stepping up the voltage is still needed a charge pump may be used instead.

      Professionally made voltage converters claim efficiency up to 95%. IIRC Ted Yapo claims efficiency of his circuit is about 80%. With supply voltage 3V and forward voltage drop 1.5V when using a red LED efficiency of a circuit not using inductor may be up to 50%; for a green LED with forward voltage 2.2V the efficiency may be 70%. It is not so large difference. In fact I am not sure if the difference will be even noticeable. Anyway total efficiency of the circuit depends much more on the LED chosen than the electrical efficiency of the circuit driving it unless the driving circuit is very poor.

      Using an inductor has considerable disadvantages:

      1. To keep efficiency high the transistor driving it must be switched off quickly. Quick turn off time means strong base/gate driver - which will be either complicated or power hungry.
      2. Time of the pulse will be probably determined by an RC circuit. Due to component tolerances the time of the pulse cannot be determined exactly. When the pulse is used to charge an inductor its inductance tolerance adds another uncertainty to peek LED current and amount of energy delivered.
      3. To keep efficiency high you want minimal resistance in the inductor - transistor - LED path. Using some form of current limiting/monitoring resistor is impossible.
      4. And finally the worst part: the inductor forces difficult compromises. To minimize switching losses the blinking frequency should be low. With low blinking frequency the amount of energy in one blink must be high enough to keep overall light output. To store enough energy in the inductor large inductance or peek current is needed. Large inductance means large DC resistance of the coil increasing loses or bulky and expensive inductor. Large peek current needs large LED which keeps high efficiency at such current - again bulky and expensive.

      On the other hand NOT using an inductor "frees" the difference between battery voltage and LED forward voltage to anything we want - such as using a resistor for current sensing, or run the oscillator from this voltage effectively using all available battery current (not energy!) for the LED.

      I think it is obvious which way I consider better!

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