2019-12-01 Oscilator Colpitts Part 2

I added a second stage to the oscillator and modulated its collector supply voltage using a transformer. The modulation waveform looks much better than previously. But the amplifier is distorting the RF sine wave. I have to work on that.


2019-11-29 Electrostatic Field Sensing, BJT Versus MOSFET

The Facebook group Electrostatic and Electromagnetic Field Detectors (I’m a member) has shown the very low current sensor made of three BC547 BJTs (bipolar junction transistors). The three BJTs are connected as Darlingtons (I called them Trarlingtons) so they multiply the base current by their betas or current gains. For three transistors, assuming each has a gain of 200, that’s 200x200x200 or 8 million! So for 8 mA to the LED, it takes only 1 nanoamp of base current!

These BJTs sense current flow and light an LED. The LED lights as long as there is a current flowing. The problem is that electrostatic fields have almost no current, that’s why they are electrostatic – they are static with a miniscule charge.

So using the three BJT sensor, if an Electrostatic field is sensed, the current flow may be so small that the LED lights too briefly to be seen or not at all. What is needed is a sensor that operates on voltage, not current. The ideal candidate is a MOSFET. Most MOSFETs are power MOSFETs, which need no gate current. But they have a gate capacitance that is hundreds or more picofarads. This is too high and a static field would not be able to charge it up enough to turn on the MOSFET. There are a few MOSFETs that are not power, just small signal, and a popular one is the 2N7000. It has a low enough gate capacitance to be turned on easily by a static field. And since it is a voltage operated device, the gate can receive a charge and turn on, and it will hold that charge for hours or days.

Here is how this super simple circuit is made. The 2N7000 source is connected to common negative (two AA or AAA cells). The anode of the LED is connected to the positive 3V. The cathode is connected to a 220 ohm resistor and the other end of this resistor is connected to the drain of the 2N7000. The gate is connected to a short length of wire, maybe 50 to 100 mm long.

I can touch a finger of my left hand to the +3V of the battery, and touch my right finger to the antenna and it will charge up the gate and the LED will light. It will stay lit for a long period when I take my finger away. If I touch the negative and the gate, it will discharge and the LED will go out and stay dark for a very long time after I remove my finger. The length of time it stays lit or dark depends on how well the gate is insulated from the source and drain. Any leakage from or to the gate will charge or discharge the gate and the LED will go on or off. So the gate and antenna must be kept clean and dry with no oily salty fingerprints to cause current to conduct.

The 2N7000 can be bought online, from sellers on eBay for example. I recommend getting 10 or more because the gate can be zapped by strong static charges. You can get a bunch for a few dollars. If you have the three BJT circuit already made up, you can add the 2N7000 source and drain in parallel with the emitter and collector of the third BJT, so you can compare how each operates.


2019-11-27 “Oscilator Colpitts” Doesn’t!

I found this schematic on a website and it’s recognizable because the author misspelled it oscilator – he left out one of the Ls (I corrected it on my copy). So I built it as shown but it wouldn’t oscillate. The values of base bias resistors R1 and R2 gave a saturated transistor. I had to increase R1 to 41k to get the base bias correct. But it still wouldn’t oscillate. I got it to oscillate by putting a 0.1 uF bypass capacitor across the emitter resistor R4. I increased it to 0.56 uF which was in my spare parts box.

It’s now putting out a decent sine wave at 456 kHz. The problem now is to modulate it with an audio tone of about 400 Hz. I injected that into the base of the transistor, and it can fully modulate the RF. But the modulation is distorted and asymmetrical. I need to try another way, maybe by modulating the supply voltage. I can also add another stage and modulate that. More experimenting is needed.

Update 11-28 – I changed the modulation to the emitter as shown in the schematic below (I forgot to remove the resistor and 400 Hz sine wave at the bottom – ignore it). The 400 Hz sine wave modulation is still asymmetrical, the bottom is higher than the top. And the top and bottom are slightly shifted like with the base modulation. I’m going to try adding a separate RF amp and modulation stage. Check out my blog for a continuation of this.


2019-11-26 My Radio As A Kid

from FB Comment Nov 26

I inherited old tube radios, I never had a boy’s radio, still don’t today. But I got a 2 transistor SupereX kit that I built. It was a crystal set with an earphone amplifier. Didn’t work very good because I lived a few miles from an AM transmitter and until sundown it was the only station I could hear across the whole band! 😱😱😱

I have a Heathkit radio somewhat similar. I used parts off it to do other things. I should try to get it together like it used to be. It needs a lot of parts.


2019-11-25 Fluke Meters, My Experiences

From conversation with Bill Sherman 2019 Nov 25

I had my new Fluke 75 for a year or so and one day I turned it to the off position but the display didn’t go off. I tried it a few times but the only way I could get it to go off was to remove the battery. I checked the pamphlet and it said it was under warranty for 3 years. So I sent it back to Fluke for repair.

When I got it back, I found instructions online to convert my 75 to a 77 by changing a jumper inside. So now it’s identical to a 77.

Then the HVAC guy at work burned out a yellow 23 – looks the same as mine. But it’s not worth repairing, I just have it for what I could salvage, probably just the case.

Then later I bought a B&K autoranging meter. I like it more because it has frequency up to 2 MHz and capacitance. And it’s auto shutoff, too. I must have a dozen cheapo $5 DMMs from Harbor Freight lying around, and I use them more than any others. 🤔🤔

The first Fluke meter I used back in the mid 1960s was a big Fluke differential voltmeter. It was like having to null a wheatstone bridge – took awhile but it was a lot more accurate than an analog meter.

The second Fluke I used was a 8600 bench DMM that a neighbor gave me in 1975 because it was dead. I had to replace a +- 15V regulator chip, and I used it until I got the 75. So both Fluke DMMs that I own have had to be repaired. 😮☹️

But the Fluke DMM that really made Fluke popular with techs was the 8040 series. I got one of those at work and used it for many years and it never had a problem. Somewhere during one of the moves we had to make it disappeared – probably stolen by someone.😣😠


2019-11-16 Fixing a Driver Transformer With An Open Primary

Also FB group Building Transistor Radios


I was working on a Realtone 10 transistor AM radio that had a driver transformer with an open primary. I slit open the wrap and found the tiny fine wire on the right was loose so I soldered it to the heavy wire. But it was still open. ☹️😱 So I checked the left wire and found it wasn’t connected, so I soldered it to the heavy wire. Checked with the DMM and it’s *still* open! I’m so sad! Buah-wuh-wah. 😭😭😭 Why is this so fouled up?? It’s crazy! Like it’s been hit by lightning! Now I’m just bummed out!

Update a few hours later – I tack soldered into the circuit a transformer with four 600 ohm windings. I connected two of the windings in series for the primary, and the other two in series with the ‘center tap’ to drive the bases of the two output transistors. The radio now works just fine – there was *nothing* wrong that could cause the driver transformer to have excessive current and burn out. What the hell caused the driver to go bad?? Some others in the FB group thought it might be the factory used corrosive solder. It seems to be a plausible answer.

Update Nov 21 – I ordered some LR-44 driver transformers from a UK seller, but they may not get here until next year. So I wired the temporary transformer into the circuit permanently, held in place with a piece of wire. This doesn’t look so hot but it works okay.

Final look at the old timer. A little worn but it’s working okay.


2019-11-15 Build A LED Tester

Big Clive’s YouTube video

Ohhh nooo! After all the trouble Big Clive went to show us how a Joule Thief works, he *digressed* to using a 9V battery for his LED tester! Horrors! 😱😱😱

I built a Joule Thief with a socket like he used for the LED. But the JT must not be left open circuited without the LED because the voltage will rise very high and can damage the transistor. So I connected a 6.8V zener in parallel with the socket so the voltage can’t rise above 6.8V. instead of a zener diode, you can use two or three LEDs in series to limit the voltage. When the led is plugged into the socket the 2 LEDs will go dark and the single LED will light up. If the LED is shorted, the two LEDs will go dark, indicating a problem.

One point: the typical 5mm LED and most others, too have a reverse voltage rating of 5V maximum, so if you insert the LED backwards, you are putting over 9V in reverse across the LED. I have never found that the LEDs I’ve tested have been damaged by this, however it is excessive voltage and should not be done.


2019-11-12 Kicad and ExpressPCB Printed Circuit Board Software

From FB group, 2019-11-12

I hope Kicad has improved in the few years since I tried it. Then it really wasn’t a single app, it was several programs that kinda sorta did something. I didn’t expect much – it was about what I paid for it. I came to the conclusion that if I wanted something decent, I would have to pay for it. Up until then I used ExpressPCB, and it did a decent job, but it was proprietary and locked me into a single vendor. But I don’t need a large quantity of PCBs, so I think that ExpressPCB was my best choice.


2019-11-07 Audio Impedance Matching Box

I’ve often had to kloodge together an audio transformer, jacks and other stuff to make a medium impedance output match a low impedance headphone. But it turns into a mess of wires and parts dangling from a transformer. I thought about what I would do to make a box with the necessary stuff, like inputs, jacks, switches and outputs and jacks. But I just never got around to putting it onto paper or other form. I found something that gets much of what I was thinking to build, except I would add a few more stereo (TRS) jacks and RCA jacks, along with switches to put the stereo to mono and to put the headphone elements in series. So there is more to the box that I want to build than is shown here, but this is a very good start. This uses the popular Bogen T725 line matching transformer, available online or one similar to it.


2019-11-05 Einstein 100 Years Ago

Einstein became famous 100 years ago.

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