Tag: Improvements

After un-wedging the astable’s synchronized periods and recharging the harvested 18650 lithium cell, I put a sock (not shown) over the radome and let it blink on a corner of the desk:

The periods are much too short and the NPN astable currents much too high, but the thing runs for about ten days before the over-discharge circuit shuts it down.

So a single NPN astable driving a single-color LED with a more reasonable period should get a month or so from an end-of-life 18650 cell and a MOSFET astable might run for two months.

I’ve been using not-dead-yet lithium batteries to power astable multivibrators blinking LEDs on the red-to-yellow end of the spectrum, because the over-discharge protection circuitry in the batteries shuts down at 2.5 V, while not eking much light from LEDs toward the blue end of the spectrum.

Back in the late 60s, when integrated circuits were new, National Semiconductor designed and, in the early 70s, introduced the LM3909: “a monolithic oscillator specifically designed to flash Light Emitting Diodes”. The IC used an electrolytic capacitor as both timing element and voltage booster by charging the cap, then switching it in reverse series with the LED, to produce a voltage drop larger than the 1.5 V battery supply. The original National Semiconductor LM3909 datasheet will get you started and Application Note 154 gives more details and insight.

Rob Paisley’s work from 2008 suggested a discrete-transistor version might look just as attractive, in a techie sort of way, as the astables, and perhaps boost the 2 V from a pair of not-dead-yet alkaline cells high enough to light a blue LED.

Some LTSpice twiddling produces a suitable circuit:

The labeled nodes correspond to pin numbers on the IC package, with a suffix indicating what they did for a living. R2 combines the two timing resistors in the IC into a single unit, so “P18-RC” combines the pins. The Q2 pair over on the right forms a current mirror driving Q3, which the doc calls the “power transistor”, to yank the positive end of the capacitor to ground to light the LED.

The LED is faked by a PT-121-B diode with a 2.34 V forward drop at 20 mA. It’s rated for 20 A average current, so it’s not a particularly good model for a piddly 5 mm LED, but I’ll define it to be Good Enough for now.

Running the simulation at 1.5 V is encouraging:

The green trace gives the voltage across the capacitor. Under these conditions, the voltage stays positive, although not by much.

Running it from a 3 V supply changes the results:

The cap charges to about the same voltage, but the pulse now lasts long enough to charge it nearly half a volt in the wrong direction. This is Bad Practice, even though my similarly offending astables have been doing it for years.

The data sheet points out that the forward drops of Q1 and Q2 determine the trigger level for the start of the LED pulse, so adding another forward-biased junction in series should let the cap charge to a higher voltage and, for the same pulse duration, pull the low end up above zero to increase overall happiness.

Lithium battery packs have overcurrent protection cutouts, but alkaline cells depend on their internal resistance and may overheat in response to a serious short circuit. So adding a PTC fuse to the circuitry over an alkaline battery case seemed appropriate:

That’s a test setup for a discrete-transistor version of an LM3909 LED blinker, about which more later. The PTC fuse looks a lot like a ceramic capacitor with one leg caught in an alligator clip.

Two bags of PTC fuses recently arrived from halfway around the planet, rated at 100 mA and 170 mA. One allegedly came from JinKe and the other probably didn’t pass through a Littelfuse factory despite its part number, but the only datasheet I can find is for the Littelfuse RXEF PTC PolySwitch series, which is surely close enough.

I set up a torture test involving a bench power supply and an ammeter, both offscreen and left to your imagination:

At 75 °F:

• 100 mA PTC – 4.75 Ω
• 170 mA PTC – 2.80 Ω

With a dead short simulated by 3 V from the supply, the current stabilized at:

• 100 mA PTC – 125 mA
• 170 mA PTC – 135 mA

Cranked to 5 V for that good old TTL vibe:

• 100 mA PTC – 70 mA
• 170 mA PTC – 85 mA

The datasheet says they’re good up to 60 V, but that’s just crazy talk.

The abuse put a shiny gloss on the epoxy coating, sort of like when you overcooked one of those wax-insulated capacitors back in the day.

Despite that, a PTC fuse is better than a dead short, if only because the plastic battery case won’t get all melty with the batteries supplying less than half a watt.