Forum Diagram

Below is a schematic diagram of a USB player.

This is a simple TV audio video transmitter circuit can be constructed using this schematic diagram . This TV audio video transmitter circuit can be used to transmit video signals from VCR ( or some other device ) to a TV without using any cable .

Video signals input at jack J1 are first terminated by resistor R6 and coupled through capacitor C1 to clamping-diode D1. Potentiometer R3 is used to set the gain of the video signal; its effect is similar to that of the contrast control on a TV set.

 

Bias-control R7 can be used to adjust the black level of the picture so that some level of signal is transmitted, even for a totally dark picture.

RF-transformer T1 and its internal capacitor form the tank circuit of a Hartley oscillator thats tuned to 4.5 megahertz. Audio signals input at J2 are coupled to the base of Q3 via C2 and R4: the audio signal modulates the base signal of Q3 to form an audio subcarrier that‚s 4.5-megahertz higher than the video-carrier frequency.

The FM modulated subcarrier is applied to the modulator section through C5 and R9.
Resistor R9 adjusts the level of the subcarrier with respect to the video signal.

Transistors Q1 and Q2 amplitude modulate the video and audio signals onto an RF-carrier signal. The operating frequency is set by coil L4, which is 3.5 turns of 24- gauge enameled wire on a form containing a standard ferrite slug.

The RF output from the oscillator (L4, C7 and C9 ) section is amplified by Q5 and Q6, whose supply voltage comes from the modulator . Antenna matching and low-pass filtering is performed by C12, C13, and L1.

Resistor R12 is optional; it is added to help match the output signal to any kind of antenna.
To align this audio video transmitter you need to tune a TV receiver to an unused channel between 2 and 6. The TV must have an indoor antenna connected directly to it; an outdoor antenna or cable wont work. Make sure both potentiometers (R3, R7) are in middle position and apply power to the transmitter. Adjust L4 with a nonmetallic tool until the TV screen goes blank ,then fine-adjust L4 for the "most-blank" picture.
Connect the video and audio outputs from a VCR(AV source) to jacks J1 and J2 (respectively) of the transmitter .

After that you should see a picture on the TV screen: if you do, readjust L4 for the best picture; if you dont, check the board for any bad connections. Next, adjust R3 for the best picture brightness and R7 for the best overall picture.

Finally, adjust T1 with a nonmetallic tool for the best sound .
The TV transmitter combines line level audio and video signals, and transmits the resulting signal up to 300 feet. The circuit can be powered from a 9-12V power supply circuit .

Most PN-junction diodes can be used as photodiodes. While not optimized for this application, they do work. When the diode is reverse biased, it will produce a small photovoltaic output as the light level is increased. LEDs are particularly suited for this task because their housings are transparent.

You can construct a simple schema that will assess the condition of ambient lighting and, because many LEDs’ packages are tinted to enhance their emitted color, may even yield a reasonable evaluation of the detected color. The results are not as effective as those obtained using a high-quality optical filter, which typically has narrow bandpass characteristics, but they can be quite acceptable. Though the design described here does not produce the accuracy of designs with laboratory-grade photodetectors and transimpedance amplifiers, it can be quickly assembled and will produce usable results at a low cost.

Three LEDs are used; experimentation will indicate which device has the best sensitivity to which color (Figure 1). The ambient light falling on the LEDs causes some current flow—typically in the range of 10 to 100 nA—through each LED, depending on the applied illumination level. This current flows through the base of a transistor, Q1, and is amplified. Q1’s collector current then splits between potentiometer R4, which acts as a first-stage gain calibration, and the base of Q2.

Photo Meter Assesses Ambient Light Schematic

Q2 provides further amplification and drives the left side of a bridge schema (D1A and D1B). Note that R2/D1 and R3/D2 form a balanced bridge. Q2’s collector current provides a slight imbalance to the bridge. The meter, M, measures this imbalance. R5 adjusts the sensitivity of the meter. Set R4 and R5 such that the meter has an appropriate deflection. R4 is useful for selecting the quiescent point; R5 is useful for adjusting the sensitivity.

Before building the schema, check whether the LEDs can be used as photo sensors. To determine whether a given LED is a good photodiode, check the voltage across the LED using a common digital multimeter set to its most sensitive range—typically 200 mV. Typical output voltage should be approximately 0.3 to 1 mV with typical office illumination.