1. Development of equipment for temperature monitoring in technological processes (VI)
Computer interface
The Politemp II system is equipped with an RS232 asynchronous serial port with which the communication with the computer (PC) is made. The communication is necessary for downloading the measured temperature values and their subsequent processing.
The RS232 serial port is not fully implemented but only strictly necessary, namely TX - RX signals and electrical ground. The system is configured to communicate at a rate of 19200bps, on 8 bits, without parity and a single stop bit.
In the case of this communication it is not necessary to control the data transfer. The electrical diagram of the RS232 serial port is presented in figure 36 of the article.
Modbus protocol
This protocol defines a message structure that industrial systems recognize and use in data transmissions, regardless of the type of communication networks used.
The protocol describes the process used by a controller to request access to another device's resources, how it responds to the access request and how transmission errors are detected and reported.
It also establishes a common format for commands / requests / responses and for data.
During communication on a Modbus network, the protocol determines how each controller is identified / addressed, how a message addressed to it is recognized, determines the type of action initiated and how the data or other information contained in the message is extracted.
If a response message is requested, the addressed controller will construct a response message using the entire Modbus protocol. The controllers communicate using the technique master-slave, in which only one device (master) can initiate transactions (requests).
2. 300W final audio amplifier with MOSFET
It presents an amplifier made with final stages based on MOS-FET transistors, which can develop a musical power of up to 600 in bridge configuration (with two such amplifiers). The assembly is done in a kit, by the Velleman company.
The amplifier is equipped with a thermal protection circuit. Class A operation gives the amplifier a very good efficiency, the power losses being negligible.
Also noteworthy is the protection and timing circuit which eliminates the noise from the speakers that appeared when the amplifier was turned on.
The speakers are also protected in the event of a direct voltage component, at the output of the amplifier.
3. mAh-meter for Ni-Cd battery elements
A useful laboratory device is also the mAh-meter, which allows measuring the capacity of Ni-Cd battery elements. An idea is presented on the mechanical (pulse) counter.
Measuring the capacity of batteries made of battery elements is a very useful operation in practice, when you want to make a decision to throw or not in the trash the elements that seem worn to the user.
Normally, a AA Ni-Cd battery has a capacity of 500… 750mA. The diagram presented in the article is a circuit that helps to measure the capacity of the batteries (1,2V elements), fully loaded in advance!
As a display element (direct mAh) is used for simplicity, an electromechanical pulse meter (at 24V) with at least 4 digits, but with small changes you can also use an electronic counter.
4. Vu-meter stereo
This power indicator is equally usable, regardless of whether the amplifier is built in mono or stereo.
In stereo configuration, the scale corresponding to each channel consists of 12 LEDs. The range of indication is in the range 0.001… 100 [W]. In the mono variant, the two scales are inserted and the range extends to the interval 0.001… 400 [W].
5. Multifunction audio-video monitor
By using a modulator for video (and / or audio) signals, any TV can be transformed into a monitor for surveillance with video cameras (in this sense you can use the "video modulator" made in the article.
A signal from the video camera is applied to the input, and the output is connected to the antenna input of the TV, switched to UHF tape).
Practically, you can use any type of modulator existing in the trade, a common TV A / N on which "dust has settled" can be successfully used as a monitor for video signals!
6. Board 16 relays controlled by PC
The experiment presented allows control of a large number of power relays (10A to 230Vac), respectively 16, by means of a dedicated software for the PC.
The interface with it is made on the parallel port (LPT). Constructively it contains two distinct blocks: the interface block and the CPU processing AT89C51 and the execution block, with power relays.
7. Thermal fuses
Thermal fuses are a modern and inexpensive means of short-circuit or overload protection of electronic or electrical circuits. They are the predecessors of the much more titrated resettable fuses or polyswitches as they are sometimes called.
Their advantage lies in the physical dimensions; can be mounted in (on) various components or electrical appliances (on the winding of mains transformers, on radiators or near the capsules of power transistors (or modules), etc.).
Most often we find them on the winding of mains transformers from the rectifiers or battery chargers, under the protective insulation (in series with the supply circuit), they protect against overload (by interruption), by heating the capsule to the temperature of the windings.
Thermal fuses cannot be reused (after they are "burned", ie after performing the actual protection function); are cheap components, which protect more expensive components, the example above being suggestive!
8. Proximity sensor
The assembly shown is a timed switch with proximity sensor, made on the basis of a optocoupler in IR, reflective type. Besides the many possible applications that can be imagined with it, the electronics of a hand dryer is the most suggestive!
The pulse generator is a relaxation oscillator made with a type comparator LM339, according to a classic scheme.
The elimination of repeated and uncontrolled starts, due to disturbing signals, is done with the circuit consisting of the monostable U2A and the counter U3. The monostable tilting circuit is a MMC4098 (or CD4098, no.) in retractable mounting with positive front trip, the delay time is about 5ms, so slightly longer than the period of the input pulses, received by the phototransistor. The counter is decadent, Johnson, with decoded outputs MMC4017 (CD4017 or 74HC4017).
