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I had an old AM radio board designed to operate with a 3V CR2032 coin cell. I want to revitalize this old junk with as few parts as possible and convert it into a USB-powered AM radio with a built-in speaker. Parts I have for this project include three caps, two resistors, a blue LED, an 1N4728 3.3V Zener, a speaker and an LM386 audio power amplifier.

schematic

simulate this circuit – Schematic created using CircuitLab

My question is that can I get away without the Zener and use the blue power indicator LED (my bach have a forward voltage of about 3V) instead like this:

schematic

simulate this circuit

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  • \$\begingroup\$ Compare the voltage/curent curve of LED and zener and you will understand why even small changes in supply voltage or current demand from load, the LED can not keep constant the voltage across R1. A parallel diode with a series resistor as a divider to ground it will works better, with excellent stability because the ratio of changes in LED current to change in diode current it is nearly constant (responds to temperature with same way). \$\endgroup\$ – GR Tech Dec 26 '14 at 22:58
  • \$\begingroup\$ LED works as a VERY soggy zener. Zener works as a VERY soggy TL431. TL431 allows any voltage from 2.5V to typically 30+V with 2 resistors. Note minimum cathode current of about 80 uA for regulation. TLV431 is 1.24V version and with lower I reg_min BUT often also lower Vmax. As shown with LED you have about 3 - 4 mA for U1 which may be OK for your application. \$\endgroup\$ – Russell McMahon Dec 27 '14 at 7:23
  • \$\begingroup\$ @RussellMcMahon The radio board works reasonably well over the entire voltage range of a CR2032 battery so a soggy zener is not a problem. I am aiming for the least parts possible. \$\endgroup\$ – Maxthon Chan Dec 27 '14 at 8:04
  • \$\begingroup\$ @GRTech That isn't too much a problem as the radio board was designed for a CR2032 battery, not too much current draw here and a really wide working voltage range. All output power is drawn directly from the 5V rail by the LM386 chip. \$\endgroup\$ – Maxthon Chan Dec 27 '14 at 8:06
  • \$\begingroup\$ If you do not value the LED light (but you probably do) replace D1 with a resistor sized to set voltage within acceptable range for all likely USB voltages. ame component count bu very slightly smaller total cost. |On the edge: Remove D1, set R1 value to drop 2V or so at mean U1 current. Larger decoupling helps. |Replace LM386 with 1 transistor amplifier. | Use high impedance "crystal earpiece" which will operate from U1 directly (long ago we used these directly with a "crystal set - no battery at all :-) ) | Rub 2 sticks together, build a coherer from carbon char and sharp point, then ... \$\endgroup\$ – Russell McMahon Dec 27 '14 at 19:31
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Yes, however you should bypass the LED with a capacitor such as 100uF electrolytic and you should expect the LED to vary in brightness a bit with the sound output.

You may not be able to get full sound power output while keeping the LED within maximum current specifications.

You could put a diode (eg. 1N4148) in series and use an emitter follower (eg. SS8050) to avoid both problems.

Or use a TL431 which costs less than CNY 50 per K in Shenzhen.

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  • \$\begingroup\$ The radio circuitry have its own bypass cap so I am not adding any. And the sound power is not a problem anyway as the speaker is driven by LM386 that draws 5V directly. Also I am eliminating parts so not gonna add the SS8050 (or 2N3904). About that TL431, the radio circuitry will not work very well at 2.5V and once again, I am not adding parts. \$\endgroup\$ – Maxthon Chan Dec 26 '14 at 18:39
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    \$\begingroup\$ LM431 can regulate at any voltage by adding two resistors. Sounds like it should work fine with the LED then. Word of the day: Muntzing \$\endgroup\$ – Spehro Pefhany Dec 26 '14 at 18:51
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    \$\begingroup\$ Yup, I am muntzing this design in the breadboard stage. And as I am already eliminating parts, the TL431 solution would at least keep the same part count as the Zener solution if not adding parts. \$\endgroup\$ – Maxthon Chan Dec 26 '14 at 19:01

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