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--- embedded:adventures:dualpower [2016/09/25 13:17] – pvela
+++ embedded:adventures:dualpower [2024/08/20 21:38] (current) – external edit 127.0.0.1
@@ Line 18: / Line 18: @@
 To duplicate the digital logic of the left side, the main idea is to tap off of the second pin with a pull-down.  In the off mode, the pull down ensures logic low.  In the on mode, the system will become logic high.  Since the gate has some low resistance, the right hand side circuit will behave like a resistor-based voltage divider, so it won't be pure logic high but something slightly lower.  How low depends on the value of R2. What's important is for the high to be high enough to trigger the digital logic of the other side.
+Since the gate has some nominal level of inductance too, the left-hand and right-hand side circuits will behave like a first-order stable feedback system.  They will have a rise-time and steady-state error that will depend on the resistor values chosen.  These properties will influence the maximum frequency that the opto-isolator can reproduce in digital form.  In effect, there is a maximum frequency that the input digital logic can operate at.  For things in the KHz range, it is usually not a problem.  Using a standard opto-isolator for I2C or SPI operation at MHz rates doesn't really work. Instead, specialized high-speed chips are needed, usually designed for specific communication application.
+==== mBed Implementation for RC Servomotors ====
+In this example, an opto-isolator is needed because the mBed uses 3.3V as the logic and also does not provide the current needed to drive a motor.  The servomotor here is an RC servomotor which uses a supply in the 4-6V range. Normally, we use a P2501 opt-isolator, however in this case the students used a different chip, the MCT6.
-We needed one because the 3.3V regulated Vout was not enough to control a servo motor, which requires 4-6V. In addition, the 5.0V USB out cannot function at the same time as the mBed. Either you are controlling your electronics with the mBed or powering them with 5.0V. This is not ideal.
 We found [[http://www.farnell.com/datasheets/1866419.pdf|this MCT6 datasheet]] to be helpful. The MCT6 chip contains 2 optoisolators. Here is how to connect the first one:
-. Connect your output wire from the mBed to pin 1.
+  - Connect your output wire from the mBed to pin 1.
-. Connect pin 2 to mBed ground with a small valued (1k) resistor.
+  - Connect pin 2 to mBed ground with a small valued (1k) resistor.
-. Connect pin 7 to external DC ground with a large resistor (10k).
+  - Connect pin 7 to external DC ground with a large resistor (10k).
-. Connect pin 8 to external DC Vcc and to the input signal wire of whatever you need to control.
+  - Connect pin 8 to external DC Vcc and to the input signal wire of whatever you need to control.
+In this described configuration, the right hand side appears to be operating in pull-up mode.
+;#;
+{{:embedded:adventures:optoisolator_mBed_RCservo.jpg?500|}}
+;#;
+;#;
+Figure 1: The MCT6 optoisolators used in a circuit.
+;#;
-{{:embedded:adventures:img_2055.jpg?200|}}
+The image is a bit messy due to the wires being everywhere, but it should be somewhat clear that the second, smaller proto-board has an opto-isolator on it.  It is acting as the bridge between the mBed side and the RC servomotor side. The blue wire at pin 8 of the MCT6 ultimately connects to the second (signal) pin of the servomotor.  The first and third (right-most) wires and pins are the power and ground. It looks to be that the third pin is ground.
+---------------
-Figure 1: The MCT6 optoisolators used in a circuit.
+;#;
+[[ Basics:Adventures | Main]]
+;#;