# Please explain the purpose of the resistors in this scenario

I came across this schematic and I am wondering why there are two different resistors on the output pins. I know that there has to be some resistor because otherwise if one pin was hight and the other low it would cause a short, but how would one determine the required values of these resistors and what other purpose could they serve? The pins output 5v.

The circuit is taken from Arduino TV out and the MCU pins are from a 5v Arduino and connect to a composite RCA cable.

• Have you done the math yet? – Ignacio Vazquez-Abrams Jul 2 '14 at 21:22
• It's two resistors joined together and without some context to this question or someone with extra sensory perception it's really hard to give an answer but maybe someone who has used this exact circuit can recognize what your question really is? – Andy aka Jul 2 '14 at 21:22
• @Andyaka I updated my question but I the reason I am asking is because I do not know what other information I would need. I measured the voltages and they are the same independent of whether one or both pins are high, only the current changes but I am pretty sure that composite only uses voltage and not current – stas Jul 2 '14 at 21:29
• It's a well known circuit in the Arduino realm. The missing information is the 75Ω load impedance of the video display. – Majenko Jul 2 '14 at 22:33
• @Majenko I was confused because I did not know about the 70 ohm resistor on the other end. – stas Jul 2 '14 at 22:46

## 2 Answers

The resistors form a pair of voltage dividers, with the third resistor being the 75Ω load of the target video system (monitor, etc):

simulate this circuit – Schematic created using CircuitLab

The voltage at $Output$ would therefore be dependent on which of the outputs are turned on.

With just the $Sync$ turned on the voltage would be (assuming a 5V MCU), $\frac{75}{1000+75}×5 = 0.35V$

With just video turned on the voltage would be: $\frac{75}{470+75}×5 = 0.69V$

With both turned on the voltage would be: $\frac{75}{319+75}×5 = 0.95V$

So you can see that by combining those outputs you can achieve the different voltages needed to describe a crude composite video waveform.

A traditional analog video signal is 1V pk-pk into 75 ohms, with black level at about 0.3V, so the sync pulses run from 0 to 0.3V and the video signal from 0.3 to 1.0V.

The MCU outputs will each provide 5Vpk-pk, so the two resistors are effectively in parallel (319 ohms). The output voltage is then 5V * 75/(319+75) = 0.95V. Not too far off.