# How to read digital LCR meter

#### gary350

##### Well-Known Member
The suffix after the part number specifies the package the part it in (e.g., T0220, TO92, SOIC-8). The data sheet is your friend.

Searching for LM2931 on Octopart.com shows some of the different options - see first picture below.

Clicking on one of the TO92 versions available at Arrow brings up the price breakdown for this part and a BUY button. See the second picture below.

Clicking the BUY button brings up the screen where you can actually purchase the part. Enter the quantity you'd like to order, and click the BUY button. Note that the screen says Overnight shipping is free. If you click any other option when checking out, shipping will not be free.

Once the desired parts are in your cart, click the cart and select check out.

View attachment 115905 View attachment 115906 View attachment 115907 View attachment 115909

I can find lots of cheap prices, when you add $10 postage to a 49 cent part the price is$10.49

Nice thing about 10% and 20% resistors, I needed a 160K resistor all I have is several 150K resistors, one resistor reads 160K on both meters. Same thing with 30K resistor all I have are several 33k resistors, 1 reads right on 30K. I test every part before I use it to make sure it works and value is right.

Last edited:

#### JonSea

##### Well-Known Member
What part of free overnight shipping do you not understand?

Never mind. You are beyond help.

#### gary350

##### Well-Known Member
What part of free overnight shipping do you not understand?

Never mind. You are beyond help.
Every time I go to ARROW they charge me postage. I will buy from them if I can get free postage?

Project is soldered except for 4.7pf cap, battery holders, and antenna.

#### JonSea

##### Well-Known Member
Every time I go to ARROW they charge me postage. I will buy from them if I can get free postage?
Makes no difference to me. Since all of their orders shipped to the US have free next day shipping, you are either not shopping at ARROW.COM or you can't follow directions.

For everybody else, Arrow offers great prices and free next day shipping on all orders to the US.

#### gary350

##### Well-Known Member
I am on ARROW now. I found LM2931 typed in quantity 5. Price 58 cents each. There is only 2 options, view cart or continue shopping. View cart shows, parts $2.93 each total$8.48. There is no option to pay??? I changed quantity to 1 now it says 1 part is \$6.07

I forgot about new year eve & new year day, no mail. My order won't be here till next week.

Last edited:

#### audioguru

##### Well-Known Member
Your coils are very different than mine, they will probably tune to aircraft frequencies and cause the RF cops to arrest you for causing an aircraft collision.

#### Attachments

• 30.2 KB Views: 5

#### gary350

##### Well-Known Member
Your coils are very different than mine, they will probably tune to aircraft frequencies and cause the RF cops to arrest you for causing an aircraft collision.
Circuit drawing says .1uh coils. I used online coil calculator to make both coils .1uh. I tested both coils on both of my meters I get crazy values. One meter reads coil too high the other meter reads coil too low. Next I did the math myself and got .100082uh round that off = .1uh

I did several experiments making coils and learn if diameter doubles the value goes up about 3 times with the same number of turns. I kept getting coils like 5 1/3 turns or 6 3/4 turns or 3 1/2 turns. I decided to make a 4 turn coil 3/16" long then keep changing the diameter until I get .1uh I learned coil diameter needs to be .280 diameter. I have machinist drill bit sets I wrapped wired around different drill bits to see what diameter coil turned out to be after metal spring back. I has to make a few coils to finally get .280 diameter. Diameter, length, number of turns, is correct it should be .1uh. Wish my meter would tell me for sure what the value is I guess I will find out if FM radio receives no signal then coils are wrong.

Last edited:

#### audioguru

##### Well-Known Member
Since the inductance is so low and the stray capacitance is so high then calculating the coils or measuring them is useless. My coils work fine and I simply guessed that they are 0.1uH, your calculated coils are so different to mine that they will not tune to the FM broadcast band. Why not copy mine and make your project so it works properly?

#### gary350

##### Well-Known Member
Since the inductance is so low and the stray capacitance is so high then calculating the coils or measuring them is useless. My coils work fine and I simply guessed that they are 0.1uH, your calculated coils are so different to mine that they will not tune to the FM broadcast band. Why not copy mine and make your project so it works properly?
I may have to copy yours. Soon as the cap and battery holder comes I think I will test is like it is. If it does not work then I change it. Is your coil 8 or 9 turns? I have no 1mm enamel coated wire. I may have a short piece of naked 1mm wire that I can paint.

#### audioguru

##### Well-Known Member
I stole some enamelled wire from the inductor on a speaker crossover network. My coils have 9 turns but I did not try tuning the C6 frequency range since there are many FM radio stations all across the FM broadcast band in my city.

#### gary350

##### Well-Known Member
Look at this circuit this is an idea, volume an gain are adjustable no clipping problem to 2N3904 the whole circuit can run on the same voltage up to 15 volts. Circuit is from LM386 datasheet. I double checked my coils with online coil calculator they are both .1uf. I built FM transmitter in college 1970 it had a range of 150 feet. I had to put radio on NO station free of static & noise to hear the transmitter signal, the whole circuit was one IC with a few parts, if you leave out the 2N3904 transistor that is basically the transmitter that I built.

Last edited:

#### audioguru

##### Well-Known Member
Gary, your FM transmitter with the LM386 IC has problems:
1) The LM386 is a power amplifier to drive an 8 ohm speaker. It is not a low noise audio opamp, it produces hiss.
2) Your circuit is missing a capacitor from the base of the oscillator transistor to ground to make it oscillate as a common-base transistor.
3) The oscillator transistor has no emitter resistor then it will fry if its hFE is high.
4) here is no voltage regulator so the radio frequency changes as the battery runs down.
5) There is no isolation between the antenna and the tuned LC circuit so the radio frequency will change if something moves towards or away from the antenna.
6) It is missing pre-emphasis so it will sound muffled with missing high audio frequencies when heard on a normal FM radio.

#### gary350

##### Well-Known Member
Gary, your FM transmitter with the LM386 IC has problems:
1) The LM386 is a power amplifier to drive an 8 ohm speaker. It is not a low noise audio opamp, it produces hiss.
2) Your circuit is missing a capacitor from the base of the oscillator transistor to ground to make it oscillate as a common-base transistor.
3) The oscillator transistor has no emitter resistor then it will fry if its hFE is high.
4) here is no voltage regulator so the radio frequency changes as the battery runs down.
5) There is no isolation between the antenna and the tuned LC circuit so the radio frequency will change if something moves towards or away from the antenna.
6) It is missing pre-emphasis so it will sound muffled with missing high audio frequencies when heard on a normal FM radio.
I copied Q3 of your circuit to this IC amp. Your Q3 has no emitter resistor or capacitor either. Your right about this amp it probably produces as much power as the transistor. A step down audio transformer between LM386 and 2N3904 might work. A different IC amp might actually work.

The IC FM transmitter I build in 1970 was a Radio IC project with plans. It looked like a big 10mm diameter metal transistor with 8 or 10 wires coming out of the bottom. It came with a circuit board all I had to do was buy parts, make the coil, solder parts to board, add battery and 12" wire antenna. Turn capacitor to a no channel station to a radio. That was the early days of ICs i never saw another IC like that one. I had a garage full of old TVs lots of free parts. I still have piles of old TVs and lots of free parts. LOL

That gave me the idea of a one IC FM transmitter. Maybe that can really be done. Tracking shows my 4.7pf caps on the way but not the battery holders.

Last edited:

#### audioguru

##### Well-Known Member
My Q3 is fed a 100MHz signal so it conducts then almost stops conducting then conducts again then almost stops conducting over and over at about 100MHz. It does not have a capacitor to ground at its base because it is an amplifier, not a common base oscillator. The tuned LC circuit at its collector has a fairly high impedance and is not a DC piece of wire like in your circuit.
Your "oscillator" transistor has no input signal because it is missing the base capacitor to ground so it is conducting DC like crazy and is frying.

With a 12V supply (15V is worse) then the base current is 11.3V/47k= 0.24mA and with and an hFE of 300 then the collector current is 0.24mA x 300= 72mA. Then the transistor tries to saturate but the very low resistance of the collector coil allows the entire 12V across it. Its heating is 12V x 72mA= 864mW but its maximum allowed is only 625mW when the ambient is not summer. Your transistor might be destroyed instantly because the LM386 has a max output current of almost 1000mA which is fed directly to the base of the transistor through C6. The maximum base current on the datasheet of the 2N3904 is only 10mA.

You do not need a power amplifier feeding a transformer, instead you need a transistor or opamp preamp circuit.

#### gary350

##### Well-Known Member
Soon as my 4.7pf capacitor & battery holders arrive I will see if I can tune the FM transmitter to a dead air spot on a FM radio. Looks to me like Q2 is the logical place to start then Q3.

I bought a 24 pack of AA batteries at Harbor Freight they are very light weight, if I squeeze a battery it squeezes flat like it is hollow inside. This is first time I bought Harbor Freight batteries. I put a battery in a wall clock it runs fine. It will be interesting to see how well they work in other things.

Last edited:

#### audioguru

##### Well-Known Member
Looks to me like Q2 is the logical place to start then Q3.
Q2 is the 100MHz oscillator that you tune to an unused radio frequency and Q3 is a buffer/RF amplifier that you tune for maximum output distance.

I bought a 24 pack of AA batteries at Harbor Freight I suspect these are AAA batteries inside a AA battery body they are very light weight, if I squeeze a battery it squeezes flat like it is hollow inside. Dollar Tree has good batteries I will buy some.
I never buy cheap junk and always buy genuine name-brand Energizer alkaline battery cells that never leak like Duracell batteries do.

#### gary350

##### Well-Known Member
4.7pf capacitors arrived today so did the battery pack for 6 batteries = 9V

Harbor Freight batteries say 1.5V but meter says 1.28 volts each battery. With 6 batteries in battery pack meter reads 7.7 volts. With batteries connected to the circuit board meter read 1.9 volts total. I am not driving 14 miles to town today for different batteries.

I have a 120 VAC inverter to 9VDC I can use for this circuit.

This circuit needs LED lights on the 9V side and 5V side. I have assortment pack of multi color LEDs what ohm resistor do they need.?

Does the transmitter have enough power to light up an LED to show it is transmitting?

Last edited:

#### audioguru

##### Well-Known Member
You need to learn about simple Ohm's Law to calculate the series resistor for an LED.
Connecting an LED to the antenna of a 100MHz FM transmitter would add stray capacitance to the output LC circuit which will untune it.
The LED would probably work fine if the transmitter operates at 1MHz.

#### gary350

##### Well-Known Member
The FM transmitter works. Adjustable capacitors are very sensitive. I need a plastic tuning screw driver I lost mine many years ago. Home made wood screw driver broke, it did not last long. I learned it is easier to set C13 in the correct general location then tune C6 to match C13. I can adjust the transmitter full range of the FM radio plus a lot more. I tested transmitter on 88.3 and 107.3. I have miss placed my package of new mics this old mic does not seem to be working well. 20 new mics from china will be here in 3 weeks maybe.