Figure out what resistance it will require to hit that current. Use Ohm’s Law and math it - voltage source minus forward voltage at the desired current, then V/I. We will hopefully have a tool for this ASAP. Make sure the resistor has a high enough power rating. Review everything to make sure it makes sense. When using resistors, you’ll need to know the power required for your project so you’ll calculate this using a LED resistor value formula like this one: P = I^2_R. What this means is that power (represented by P) is equal to the current (Represented by I) in amps flowing through the circuit, which is squared and multiplied by the resistor The formula to calculate resistance in a circuit is: R=V/I or, more relevant to what we're doing: (Source Volts - LED Volts) / (Current / 1000) = Resistance*. So if we have a 12v battery powering a 3.5V 25mA LED our formula becomes: (12 - 3.5) / (25 / 1000) = 340ohms. Step 4: Ensure the Load Equalizer Wires can Reach. Once you’ve found your turn signal bulb wires, make sure your load equalizer can reach an internal metal surface of your vehicle and that the wiring can reach the turn signal wires. If not, the load equalizer wires can be extended to reach from the mounting location to the turn signal wiring. Figure 1. Simple circuit with multiple ammeters and voltmeters. Figure 2. The same circuit with the resistor and LED positions swapped. In Figure 1 we have a simple 12 V battery powering an LED. The LED needs about 10 mA to light brightly and at that current will have nearly 2 V across it. That means we need a resistor to drop the other 10 V. This calculator is based on the Ohms Law Calculator, but takes into consideration the voltage drop from the LED. To use the calculator, enter any three known values and press "Calculate" to solve for the others. Click "Calculate" to update the fields with orange borders. Calculate resistor values for LEDs using this simple calculator. The NAND gates do an AND operation with the value and that of an NE555 that is used as debounce for a switch. All components are fed from a 5V voltage source. I read about combing different logic families and am a bit confused about when to use resistors and how many: I read connecting TTL to CMOS I need a pull up resistor. Do NOT short the resistor. A resistor is need to allow the LED to operate at its forward voltage. Without a resistor the LED is forced to operate at the battery voltage putting stress on the LED. It is a myth that it's okay to connect an LED(s) directly to a battery. Below is 3 deep blue LEDs at 10 mA. The rated wattage of the resistor is: P = VI = x 0.20A = 0.112. The rated watts for the resistor needs to be at least 1/10 w, but a more standard number is ¼ watt, thus we can use a quarter watt 280-ohm resistor. Higher wattage resistors will also work. Light Emitting Diodes (LEDs) are diodes that emit light when energized. (Whenever using a resistor on an LED it should get placed before the LED on the positive electrode). Low and behold, the LED lit up once again. The 150 ohm resistor stopped enough of the 4.5V power supply from reaching the 1.7V LED that it lit up safely and kept it from burning out. XVbhKD7.