Touch switch inputs. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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Sometimes, when designing low-frequency amplifying radio equipment, the use of widely used switches of the P2K type is not advisable. In most cases, this is due to the need to connect the input connectors located on the back of the device to the switches located on the front panel with shielded wires. In this case, the total length of such wires often turns out to be unreasonably large, which reduces the noise parameters of the amplifiers. The problem can be solved only with the help of remote-controlled switching.
Many electronic switches have now been developed that can be located in close proximity to the input jacks, and their controls displayed on the front panel. In this case, the length of the wire with a low-current signal is sharply reduced. However, the use of electronic devices with switching transistors, including field-effect ones, introduces intermodulation distortion into the audio signal. The devices, where electromagnetic relays are used as switching elements, are completely devoid of this drawback.
The device offered to the attention of amateur designers is simple due to the use of thyristors and special sensor pads. The signal is switched using small-sized electromagnetic relays with two contacts. To use thyristors without selecting them according to such a parameter as "control current", it is necessary to make special touch pads. These sites should provide a dielectric gap between the metal touch plate and the contact from the control electrode of the thyristor of the order of 0,5 - 1 mm. With a light touch on the sensor pad, the electrical contacts of the relay should switch.
Ris.1
The device, the schematic diagram of which is shown in fig. 1 provides switching of only one of the three thyristors. With the help of capacitors C1 - C3, a blocking negative potential is created on the anodes of the remaining thyristors. The switching diagram of the relay contacts is shown in Fig.2.
Ris.2
The device uses MBM, KM6-B type capacitors, MLT type resistors. All relays are of the RES-60 type (passport RSCh.569.438). In extreme cases, it is possible to use two relays of the RES-10 type connected together (passport RSC.591.006). True, in this case, the current consumed by the device increases. The use of other types of relays is limited by the maximum allowable anode-cathode current of the applied thyristors (75 mA).
An input switch assembled from serviceable parts does not require adjustment. The device can be easily turned into a pseudo-sensor if closing buttons (for example, KM-1) are used, which supply a control pulse to the thyristors from the +15 V bus through the current-limiting resistor R4.
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