Divergence Meter Battery Mod

The divergence meter I built can be powered by an internal 9V battery. However, a regular 9V battery doesn’t last very long (between 10 and 45 minutes). LiPo 9V batteries last much longer – over 3 hours, though they aren’t really 9V as they are actually two regular LiPo batteries in series – so the actual voltage is at most 8V, dropping to 6V just before the power runs out.

However, that still means that you have to open the clock up to change the battery when it is out of power. A better alternative would be to include a power-sharing charger inside the case, so that the LiPo will charge up when the clock is plugged back into the power. A quick(ish) search of the internet turned up this 9V LiPo charger module, which is small enough to fit inside the case along with the battery. So that gave me the charger, but I needed to add the load-sharing part. A load-sharing charger will use some of the input power (input from the wall adapter) to charge up the battery, while the clock runs off the rest. It also needs to disconnect the battery from the clock as long as the clock is plugged in to the wall adapter, but instantly connect the battery to the clock as soon as it is unplugged from the wall adapter. This is actually pretty easy, and my circuit is basically the one described here.

Here is the basic circuit:

Load Sharing Charger

The load-sharing part is inside the dashed box. Here is what my implementation of this looks like. I used a surface-mount MOSFET, because I happened to have one already:

Load Sharer

This works as follows:

  1. As long as there is power on Vin, the P-channel MOSFET (Q1) will be turned off, preventing any power from being discharged from the battery and also preventing any power from Vin being directly applied to the positive terminal of the battery. Power from Vin will flow to V+ through the Schottky diode (D1).
  2. While there is power on Vin, the TP5100 module will also be able to charge the battery, if necessary.
  3. As soon as power is removed from Vin, the 100K resistor (R1) will pull Vin to ground, which will turn on the MOSFET allowing current to flow from the battery to V+.
  4. This is why the Schottky diode (D1) is needed. It stops the gate of the MOSFET being pulled high again by the battery, which would turn it off, etc. The only reason that D1 is a Schottky diode, is to minimize the voltage drop from Vin to V+.
  5. The LED (D2) shows when the battery is being charged.

I checked it all out before stuffing it into the case:

Load Sharing Charger

To integrate this with the clock, you have to cut the track from the Vin pin on the barrel connector (power in), then wire that to the Vin connector on the battery charger. Then wire V+ from the load-sharing board to the other side of the track that you cut – just find an easy place on the board to do that.

To re-use the existing switch, remove the optional diode (if you installed it) and just wire the switch up as shown in the circuit diagram above.

Finally, if you choose to use the charging indicator LED, you will have to drill a hole in the case somewhere to install it.

Here it is in the case:

Charger in the Case
View Showing LED Placement

I took a time-lapse video of the clock running off battery. As you can see, it lasts a little over three hours:

Display the Time on an Old Frequency Counter

A while back I bought an old frequency counter that has a Nixie tube display. It is a Japanese SF-87A made by the Sansei Electronics company. I bought it for the tubes – eight CD66 – however, when it arrived, it was in very good condition and worked just fine, so I decided not to harvest the tubes from it. I subsequently used it to display the frequency of my Nixie power supply, but that was just looking for an excuse, I didn’t actually need to use it for that:

Displaying the frequency of my Nixie power supply.

This left me in a bit of a quandry, but not too long ago Alic Loeliger suggested on the Nixie Clocks Fan Page on Facebook that I could get it to display the time by just sending the right number of pulses to it in a given time. This was such a simple idea that I had to try it.

The maximum gate time of the counter is 1 sec, and the largest number I would need to display would be 125959 (aka 12:59:59) for a 12-hour clock, or 235959 (aka 23:59:59) for a 24-hour clock. In other words, I would need to generate (at most) either a 126KHz signal, or a 236KHz signal. As I was doing this in software, because I am a software engineer, I went for the lower of the two numbers – I wanted to make sure that one of the processors I had available would be able to do it. I had a choice of an Arduino Uno, or an ESP8266. I ended up going with the ESP8266, which I programmed using the Arduino ESP8266 toolset.

At first, I split the number of pulses evenly over the gate time. i.e. if I needed to display 10:00:00, I would send send one pulse every 0.00001s. Then I realized that the counter just counted the number of pulses that occurred within the gate time, then divided that count by the gate time to get the frequency.  In other words, all I had to do was generate the right number of pulses in less than one second. However, when I tried this, it didn’t work – I assume the pulses were too close together for the counter to detect them. So, back to sending them spread over the gate period.

The result is kind of, sort of OK. The higher the frequency gets, the less able it is to accurately display the seconds – there is some ’rounding’ involved. Here is a video of it displaying 03:07:21 – note I have no control over the placement of the thousand separators:

Here is the source code:

#include "Arduino.h"
#include 

// Use pin 0
#define PIN 0

volatile unsigned long scaledPeriod = 0;
unsigned long oldMicros = 0;
unsigned long counter = 0;

/*
 * generate the pulse train in a timer interrupt. The function uses
 * the period, rather than the frequency. This is in 1/100,000,000s
 * units
 */
const int counterPeriod = ESP.getCpuFreqMHz() * 10;

void ICACHE_RAM_ATTR isr() {
    uint32_t ccount;
    __asm__ __volatile__("esync; rsr %0,ccount":"=a" (ccount));
	timer0_write(ccount + counterPeriod);

	unsigned long newMicros = micros();
	// micros is millionths of a second. Our period is in 
	// 100,000,000 of a second, so we multiply the difference
	// by 100
	unsigned long diff = (newMicros - oldMicros) * 100;
	if (scaledPeriod > 0 && (diff > scaledPeriod)) {
		digitalWrite(PIN, HIGH);
		oldMicros = micros();
	} else {
		digitalWrite(PIN, LOW);
	}
}

void setup()
{
	// Set to some arbitrary time
	setTime(3, 7, 20, 1, 2, 2018);
	pinMode(PIN, OUTPUT);
	timer0_isr_init();
	timer0_attachInterrupt(isr);

	timer0_write(ESP.getCycleCount() + counterPeriod);
}

unsigned long  oldTime = 0;

void loop()
{
	unsigned long newTime = hour() * 10000 + minute() * 100 + second();

	if (newTime != oldTime) {
		oldTime = newTime;
		if (newTime != 0) {
			// Need a numerator that is >> newTime could be
			scaledPeriod = 100000000/newTime;
		} else {
			scaledPeriod = 0;
		}
	}
}

This code has some problems! As newTime gets larger, scaledPeriod loses more and more precision. Furthermore, because the interrupt routine is called at a fixed periodicity, it is essentially sampling the waveform. Both of these things mean that the displayed time gets less accurate as the absolute size of the number being displayed is increased.

In retrospect, it would be better to adjust the periodicity of the interrupt routine to match that of the waveform we are trying to generate. That would be an improvement, but we would still have problems caused by the rather coarse granularity of the timer we have available.

Still, the result isn’t too bad.