High Quality Intercom
This circuit consists of two identical intercom units. Each unit contains a power supply, microphone preamplifier, audio amplifier, and a Push To Talk (PTT) relay circuit. Only two wires are required to connect the units together. Due to the low output impedance of the microphone preamplifier, screened cable is not necessary, and ordinary two-core speaker cable or bell wire may be used. The schematic can be divided into four parts: power supply, microphone preamplifier, audio amplifier, and PTT circuit. The power supply is designed to remain on continuously, which is why no on/off switch is included. A standard 12 V RMS secondary transformer rated at 12 VA will power the unit. Fuses are provided at both the primary and secondary inputs, before the rectifier. The 1 A fuse must be a slow-blow type to handle the peak rectifier current as the power supply electrolytics charge from zero volts. The microphone amplifier is a two-transistor direct-coupled amplifier. BC108B transistors can be used interchangeably with BC109C transistors. The microphone utilized is a three-terminal electret condenser microphone insert, which is popular and requires minimal current to operate. The preamplifier, also shown in the audio circuit section, features high gain and low distortion. The last transistor is biased to approximately half the supply voltage, providing maximum overload margin for loud signals or voices. Gain can be adjusted with a 10 kΩ preset. Sensitivity is very high, allowing even distant sounds, such as a ticking clock, to be heard through the loudspeaker. The audio amplifier is based on the widely used National Semiconductor LM380, which requires only a 50 mV input to deliver 2 W RMS into an 8-ohm loudspeaker. The choice of loudspeaker affects overall sound quality; a small loudspeaker may lack bass response. An old 8-inch radio loudspeaker was used in testing. The 4.7 µF capacitor at pin 1 of the LM380 helps filter out any mains hum from the power supply and can be increased to 10 µF for improved power supply rejection. The PTT circuit employs a simple SPDT relay to switch between the microphone preamplifier output and loudspeaker input. The normally closed contact allows each intercom unit to be in a "listening" mode. The non-latching push-button switch must be held to communicate. A 100 µF capacitor across the relay serves two purposes: it protects the semiconductors from the relay's back EMF and delays the relay's release, preventing the last word from being cut off. This circuit does not include a "call" button, as it is intended to remain on continuously, allowing users to hear each other when speaking from either unit. Setup is straightforward; the volume should be set to a comfortable level, and the microphone preset adjusted while speaking at a normal volume from one meter away. Proximity to the microphone is not necessary, as it can capture conversations from anywhere in a room. If the units are positioned far apart, the cable may pick up hum or radio interference. To mitigate this, twisted pair cable can be employed, as each successive turn cancels interference from the previous one. Additionally, a small capacitor, such as 100 nF, can be placed between the common terminal of each relay and ground to shunt high-frequency signals to earth. Alternatively, a low-value resistor of approximately 1 kΩ can be used to shunt interference and hum, although this will also affect the speech signal. However, due to the low output impedance of each microphone preamplifier and the low levels of speech signals, this will have minimal impact on audio clarity while reducing interference to an acceptable level. The pinout of the LM380, viewed from above, is represented in the schematic as a triangle, with unconnected pins marked as "NC."This circuit consists of two identical intercom units. Each unit contains a power supply, microphone preamplifier, audio amplifier and a Push To Talk (PTT) relay circuit. Only 2 wires are required to connect the units together. Due to the low output impedance of the mic preamp, screened cable is not necessary and ordinary 2 core speaker cable, or
bell wire may be used. The schematic can be broken into 34 parts, power supply, mic preamp, audio amplifierand PTT circuit. The power supply is designed to be left on all the time, which is why no on / off switch is provided. A standard 12 V RMS secondary transformer of 12VA will power the unit. Fuses are provided at the primary input and also secondary, before the rectifier. The 1 A fuse needs to be a slow blow type as it has to handle the peak rectifier current as the power supply electrolytics charge from zero volts.
The microphone amplifier is a 2 transistor direct coupled amplifier. BC108B transistors will work equally well in place of the BC109C transistors. The microphone used is a 3 terminal electret condenser microphone insert. These are popular and require a small current to operate. The preamp is shown in my audio circuit section as well, but has a very high gain and low distortion. The last transistor is biased to around half the supply voltage; this provides the maximum overload margin for loud signals or loud voices.
The gain may be adjusted with the 10k preset. Sensitivity is very high, and a ticking clock can easily be heard from the distantloudspeaker. The amplifier is based on the popular National Semiconductor LM380. A 50 mV input is all thats required to deliver 2W RMS into an 8 ohm loudspeaker. The choice of loudspeaker determines overall sound quality. A small loudspeaker may not produce a lot of bass, I used an old 8 inch radio loudspeaker. The 4. 7u capacitor at pin 1 of the LM380 helps filter out any mains hum on the power supply. This can be increased to a 10u capacitor for better power supply rejection ratio. The push to talk (PTT) circuit is very simple. A SPDT relay is used to switch between mic preamplifier output or loudspeaker input. The normally closed contact is set so that each intercom unit is "listening". The non latching push button switch must be held to talk. The 100u capacitor across the relay has two functions. It prevents the relays back emf from destroying the semiconductors, and also delays the release of the relay. This delay is deliberate, and prevents any last word from being "chopped" off. This circuit does not include a "call" button. This is simply because it is designed to be left on all the time, someone speaking from one unit will be heard in the other, and vice versa.
Setup is simple, set to volume to a comfortable level, and adjust the mic preset while speaking with "normal volume" from one meter away. You do not need to be in close contact with the microphone, it will pick up a conversation from anywhere in a room.
If the units are a long way away, there is a tendency for the cable to pick up hum, or radio interference. There are various defenses against this. One way is to use a twisted pair cable, each successive turn cancels the interference from the turn before.
Another method is to use a small capacitor of say 100n between the common terminal of each relay and ground. This shunts high frequency signals to earth. Another method is to use a low value resistor of about 1k. This will shunt interference and hum, but will shunt the speech signal as well. However as the output impedance of each mic preamp is low, and the speech signals are also low, this will have little effect on speech but reduce interference to an acceptable level.
The LM380 pinout viewed from above is shown below on the left. In the schematic, the LM380 has been represented as a triangle, the pins are shown on the right hand diagram. Pins marked "NC" have no connection and are not used. Corey Rame 🔗 External reference
bell wire may be used. The schematic can be broken into 34 parts, power supply, mic preamp, audio amplifierand PTT circuit. The power supply is designed to be left on all the time, which is why no on / off switch is provided. A standard 12 V RMS secondary transformer of 12VA will power the unit. Fuses are provided at the primary input and also secondary, before the rectifier. The 1 A fuse needs to be a slow blow type as it has to handle the peak rectifier current as the power supply electrolytics charge from zero volts.
The microphone amplifier is a 2 transistor direct coupled amplifier. BC108B transistors will work equally well in place of the BC109C transistors. The microphone used is a 3 terminal electret condenser microphone insert. These are popular and require a small current to operate. The preamp is shown in my audio circuit section as well, but has a very high gain and low distortion. The last transistor is biased to around half the supply voltage; this provides the maximum overload margin for loud signals or loud voices.
The gain may be adjusted with the 10k preset. Sensitivity is very high, and a ticking clock can easily be heard from the distantloudspeaker. The amplifier is based on the popular National Semiconductor LM380. A 50 mV input is all thats required to deliver 2W RMS into an 8 ohm loudspeaker. The choice of loudspeaker determines overall sound quality. A small loudspeaker may not produce a lot of bass, I used an old 8 inch radio loudspeaker. The 4. 7u capacitor at pin 1 of the LM380 helps filter out any mains hum on the power supply. This can be increased to a 10u capacitor for better power supply rejection ratio. The push to talk (PTT) circuit is very simple. A SPDT relay is used to switch between mic preamplifier output or loudspeaker input. The normally closed contact is set so that each intercom unit is "listening". The non latching push button switch must be held to talk. The 100u capacitor across the relay has two functions. It prevents the relays back emf from destroying the semiconductors, and also delays the release of the relay. This delay is deliberate, and prevents any last word from being "chopped" off. This circuit does not include a "call" button. This is simply because it is designed to be left on all the time, someone speaking from one unit will be heard in the other, and vice versa.
Setup is simple, set to volume to a comfortable level, and adjust the mic preset while speaking with "normal volume" from one meter away. You do not need to be in close contact with the microphone, it will pick up a conversation from anywhere in a room.
If the units are a long way away, there is a tendency for the cable to pick up hum, or radio interference. There are various defenses against this. One way is to use a twisted pair cable, each successive turn cancels the interference from the turn before.
Another method is to use a small capacitor of say 100n between the common terminal of each relay and ground. This shunts high frequency signals to earth. Another method is to use a low value resistor of about 1k. This will shunt interference and hum, but will shunt the speech signal as well. However as the output impedance of each mic preamp is low, and the speech signals are also low, this will have little effect on speech but reduce interference to an acceptable level.
The LM380 pinout viewed from above is shown below on the left. In the schematic, the LM380 has been represented as a triangle, the pins are shown on the right hand diagram. Pins marked "NC" have no connection and are not used. Corey Rame 🔗 External reference
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