Ohmmeter from the indicator from the tape recorder. Connecting a VFD indicator from an old Soviet tape recorder to a computer. Why one device cannot measure a wide range of values
ХР1 R1 Ш R2 * 51X
How to "stretch" a voltmeter scale. Controlling some kind of tension. sometimes it is necessary either to monitor its fluctuations, or to measure more accurately. For example, when operating an automobile battery it is important to follow * and the change in its voltage in the range 12 .. L 5 V. It is this range that it would be desirable to place on the entire scale of the voltmeter dial indicator. But. as you know, the counting on any of the ranges of almost all measuring instruments comes from zero value and it is impossible to achieve a higher readout accuracy in the area of interest.
Nevertheless, there is a way to "stretch" almost any part of the scale (beginning, middle, end) of the voltmeter direct current... To do this, you need to use the PROPERTY of the zener diode to open at a certain voltage equal to the stabilization voltage. For example, to stretch the end of the scale of the 0 ... 15 V range, it is enough to use a zener diode in the same role as in the previous experiment.
Take a look at fig. 4. Zener diode VD1 is connected in series with a single-limit voltmeter composed of a pointer indicator PA1 and an additional resistor R2. As in the previous experiment, the zener diode “eats up” a part of the measured voltage, which is equal to the stabilization voltage. As a result, a voltage exceeding the stabilization voltage will be supplied to the voltmeter.
IRADISG-BEGINNERS "_
This voltage will become a kind of zero reference, and this means that only the difference between the highest measured voltage and the stabilization voltage of the Zener diode will "stretch" on the scale.
The device shown in the figure is designed to control the battery voltage in the range from 10 to 15 V. But this range can be changed at will by appropriate selection of a zener diode and resistor R2.
What is the purpose of R1? In principle, it is not required. But without it, while the zener diode is closed, the arrow of the imdi cator remains at the bullet mark. The introduction of a resistor allows observing a voltage of up to 10 V at the initial section of the scale, but this section will be strongly "compressed".
After collecting the parts shown in the diagram and connecting them with the pointer indicator PA1 (micro ammeter M2003 with a full deflection of the arrow of 100 μA and an internal resistance of 450 Ohm), connect the XP1 and XP2 probes to a power supply with an adjustable output voltage. Smoothly increasing the voltage to 9 ... 9.5 V, notice a slight deviation of the indicator arrow - just a few divisions at the beginning of the scale. As soon as with a further increase in voltage it exceeds the stabilization voltage, the angle of deflection of the arrow will sharply increase. Approximately from a voltage of 10.5 to 15 V, the arrow will pass almost the entire scale.
To verify the role of the resistor R1, turn it off and repeat the experiment. Until a certain input voltage, the indicator needle will remain at zero.
You might be interested in this way of "stretching" the scale and want to practically implement it to control other voltages. Then you have to use the simplest calculations. The initial data for them will be the voltage measurement range (l) m> x), the total deflection current of the indicator arrow (11Pax), the starting point current (1pc) and the corresponding reference voltage (UIIljn).
For example, “let's calculate * our device shown in the diagram. Suppose that the entire fabric of the device CImex = 100 μA) is intended to control voltages from 10 to 15 V, but the reference point will start from the division corresponding to the current YumkA (1N) P = 10 μA), which means that the voltage is 10.5 V (Urnin = = 10.5 V).
First, we determine the coefficients p and k, which will be needed for subsequent operations:
P = lmi „/ ln,“ = 10/100 = 0.1; k = Um, „/ Un,„> =) 0 S / 15 = 0.7.
Calculates the required stabilization voltage of the future zener diode:
UrT = Uninx (k-p) / (l-p) =
15 * 0.6 / 0.9 = 10 V.
Zener diodes D810 and D814V have such a voltage (see the reference table in the article "Zener diode").
We determine the resistance of the resistor R2 in kilo-ohms, expressing the current in milliamperes. R2 = U, nax (l-K) / lmils (l-p) =
15.0.3 / 0.1-0.9 = 50 kΩ.
In general, from the obtained value, one should subtract internal resistance an arrow indicator (450 ohms), but this does not have to be done by the resistance of the resistor R2, because it is selected practically when setting up a voltmeter.
In conclusion, the resistance of the resistor R1 is determined: Rl = Uer / p.lmax = 10 / 0.1 = = 1000 kΩ = 1 MΩ.
V. MASLAYEV
Zelenograd
The other day I was reminded of another idea for modding a computer. It will be about how to connect a fluorescent (VFD) indicator from a Soviet tape recorder to a computer.
Once upon a time, a long time ago, I had a Mayak 240-C1 tape recorder. Due to obsolescence, the tape recorder was scrapped. All that was valuable from him was an electroluminescent indicator, which was lying there, gathering dust. Once, a couple of years ago, I already tried to install it in a computer, but it did not fit the design.
The indicator looks like this:
And today I will tell you how to connect such or a similar indicator to a computer.
So, let's start with a schematic diagram:
but we do not need the whole scheme, we are only interested in a part
As you can see in the diagram, the indicator has double power: bipolar ± 15 volts and alternating 5 volts. But the indicator remains operational when powered by a bipolar voltage of ± 12 volts and a constant voltage of + 5 volts.
Connect XP1 as follows (designations according to the diagram):
1 - zero
2 - +5
3 - +12
4 - -12
5 - zero
To make it more convenient to connect, I took a non-working and half-soldered motherboard
and soldered the wires with back side ATX connector and plugged in the power supply.
Now, when the power is supplied to the indicator, it is necessary to send some signal to it. I will use an mp3 player as a signal source.
The XP2 wiring diagram is very simple (designations according to the diagram):
1 - left channel
2 - right channel
3 - indicator of the type of tape Fe
4 - indicator of the noise reduction system PSh
5 - indicator of the type of Cr strip
6 - microphone on indicator
7 - loudspeaker on indicator
8 - recording indicator
Taking out a cable from my inventory to connect a CD-ROM drive to a sound card
And after removing the native connectors from it, I soldered one end to the indicator board, and soldered a 3.5mm jack to the second
In general, this gray cable is a very good help in such cases, because inside the insulation there is a shielded two-channel stranded wire and is long enough for many applications. That's just, unfortunately, in recent times, very often these cables are not shielded. But something I digress, we continue.
♦ In the previous article: To control the charging current, a ammeter for 5 - 8 amperes.
An ammeter is a rather scarce thing and you cannot always pick it up for such a current. Let's try to make an ammeter with our own hands. 
To do this, you will need a dial gauge of the magnetic-electric system for any current of the full deflection of the arrow on the scale.
It is necessary to make sure that it does not have an internal shunt or additional resistance for the voltmeter.
♦ The measuring pointer has an internal resistance of the moving frame and a full deflection current of the pointer. A pointer device can be used as a voltmeter (additional resistance is connected in series with the device) and as an ammeter (additional resistance is connected in parallel with the device).
♦ Scheme for the ammeter on the right in the figure.
Additional resistance - shunt calculated according to special formulas ... We will make it in a practical way, using only a calibration ammeter for current up to 5 - 8 amperes, or by using a tester, if it has such a measurement limit.
♦ Collect uncomplicated scheme from a charging rectifier, a reference ammeter, a shunt wire and a rechargeable battery. See picture ... 
♦ Thick steel or copper wire can be used as a shunt. It is best and simpler to take the same wire as the secondary winding was wound, or a little thicker.
It is necessary to take a piece of copper or steel wire with a length of about 80 centimeters, remove the insulation from it. At the two ends of the segment, make rings for bolting. Connect this segment in series with a reference ammeter.
Solder one end from our dial gauge to the end of the shunt, and draw the other along the shunt wire. Turn on the power, set the charge current by the control ammeter with the regulator or toggle switches - 5 amperes.
Starting from the place of soldering, draw the other end from the dial gauge along the wire. Set the same readings of both ammeters. Depending on the resistance of your dial gauge frame, different dial gauges will have different shunt wire lengths, sometimes up to one meter.
This is of course not always convenient, but if you have free place in the case can be neatly placed.
♦ The shunt wire can be wound into a spiral as shown, or otherwise, as appropriate. Stretch the coils a little so that they do not touch each other or put on rings made of PVC tubes along the entire length of the shunt. 
♦ You can preliminarily determine the length of the shunt wire, and then instead of the bare wire, use the insulated wire and wind it in bulk onto the workpiece.
It is necessary to select carefully, doing all the operations several times, the more accurate the readings of your ammeter will be.
The connecting wires from the device must be soldered directly to the shunt, otherwise there will be incorrect indications of the device arrow.
♦ The connecting wires can be of any length, and therefore the shunt can be located anywhere in the rectifier body.
♦ It is necessary to match the scale to the ammeter. The scale at the ammeter for measuring direct current is uniform.
For a visual assessment of the strength of the charging current, I need a device for measuring the strength of the current - an ammeter. Since there was nothing sensible at hand, we will use what we have. And this "what is" - a common indicator from old soviet radio tape recorders. Since the indicator reacts to very small currents, it is necessary to make a shunt for it.
Shunt- This is a conductor with a certain specific resistance, which is connected to the current meter device in parallel. At the same time, he passes through himself or shunts most of the electric current... As a result, the rated current calculated for it will pass through the meter device. To understand how currents flow in the nodes of the circuit, we study Kirchhoff's laws.
In order to calculate the shunt for the ammeter, I need some parameters of the measuring head (indicator): frame resistance ( Rram), the value of the current at which the indicator needle deviates as much as possible ( Iind) and the upper value of the current that the indicator should measure in the future ( Imax). For the maximum measured current we take 10 A. Now we need to determine Iind, which is achieved experimentally. But for this you need to assemble a small electrical circuit.
Using the resistor R1, we achieve the maximum deviation of the indicator arrow and take these readings from the tester PA1... In my case Iind = 0.0004 A. Frame resistance Rram we also measure with a tester, which was 1 kOhm. All parameters are known, it remains now to calculate the resistance of the shunt of the ammeter (indicator).
The calculation of the shunt for the ammeter will be performed according to the following formulas:
Rsh = Rram * Iind / Imax; we get Rsh = 0.04 Ohm.
The main requirement for shunts is their ability to pass currents that do not cause strong heating, i.e. have norms for the density of electric current for conductors. Various materials are used as shunts. Since I don’t have “miscellaneous material” at hand, I will use good old copper conductor.
Further, based on the fact that Rsh = 0.04 Ohm, according to the reference book of specific resistances of copper conductors, we select the appropriate size of a piece of copper wire. The larger the diameter, the better, but the length of the copper wire increases. I will “forget” these requirements and choose a meter segment. The main thing for me is that my shunt does not melt, especially since I will not rape it more than 6A. I twist the selected copper conductor into a spiral and solder it parallel to the measuring head. That's it, the shunt is ready. Now it remains to more accurately adjust the shunt resistance and calibrate the meter scale. This is done experimentally.
Actually, the devices. Weedon is not very much, what is already there ...
Modelers often bring up the topic on the pages of "Park Flyer" operational verification serviceability of the RU transmitter and its antenna, which is the most important point in the reliability of the interaction between the transmitter and the receiver during the flights of RU models.
To check the health of the transmitter and its antenna, I use a simple homemade Indicator electromagnetic field, which was made from a dial indicator of the recording level from an old tape recorder. The indicator turned out to be very small, smaller than a matchbox and easily fits into the breast pocket of a shirt, which allows you to monitor the transmitter's radiation and the serviceability of its antenna at any time right in the field.
The dial indicator of the tape recorder is a microammeter with a deflection current of 50 ... 100 μA.
For the manufacture of the Indicator, in addition to the head, two microwave diodes are needed, I used KD514A diodes. A half-wave piece of a suitable wire Ф 1 mm is used as an antenna. For 2.4 GHz RC transmitters, the cut length is 60 mm. The scheme of the device is very simple.
Solder the diodes to the indicator terminals. This is how the KD514A diodes look like.
Ready device.
The antenna is glued with epoxy not directly to the indicator body, although it is made of plastic, but through a piece of the rail. The fact is that the scale of the instrument is drawn on metal plate, which is attached to the back cover inside the case, and if the antenna is glued directly to the cover, then it will be located in the immediate vicinity of the metal scale at a distance of 1.5 mm from it, separated by the bottom of plastic. As a result, a small capacitance arises between the metal scale and the antenna (but the frequency is 2400 MHz!), Which decently reduces the sensitivity of the indicator - the arrow deviates by a smaller angle, and if you make a gap of 6 ... 8 mm, then the capacitance becomes negligible and the arrow deviates by large angle. Therefore, I had to make a gap from a piece of rail. Such a nuance emerged during the manufacture of the Field Indicator. 
Here is a video showing practical use Indicator.
For the manufacture of the Field Indicator, any microammeter for a current of 50 ... 100 μA is suitable, not necessarily from a tape recorder. This will only affect the dimensions of the appliance.
Here are some good M4206 100 μA heads, but these are hard to find at this time.
You can use other microwave diodes, for example: KD503, D403, D405, D605, D20.
A good microwave diode is obtained from a GT346 transistor with a collector closed to the base.
It stands in the ancient SKD-24, is quite sensitive and works up to 2.4 GHz and higher.
Successful flights and soft landing to everyone!