|more fun with coils|
Message #15 Posted by Ellis Easley on 28 July 2002, 12:44 a.m.,
in response to message #10 by Tony Duell (UK)
Regarding the transformer coupled keyboard, in a slightly related way, I've got an HP3480A nixie tube digital voltmeter which has BCD outputs and remote control inputs which are isolated with transformer coupling. The transformers are implemented with ordinary axial lead inductors. The hot side "windings" are all on one PCB and the other side on another PCB. The inductors are laid out so when the PCBs are in place, they are close enough together to couple. To achieve the required spacing, or for other mechanical reasons, the PCBs are so far apart that the inductors on one side have long leads holding them up off the PCB. To reduce the number of transformers required, some logic serializes several bits to one transformer, then de-serializes them on the other side.
I have another much more recent meter, HP3456A, which also uses transformer coupling between a guarded (mostly) analog chassis and a digital display and I/O chassis. HP uses the terms "inguard" and "outguard". In this case, each end has two toroidal inductors mounted on the PCB. A four wire harness goes from one PCB to the other. The two pairs of wires go onto the PCB at each end but each pair only connects to the ends of a short length of wire that passes through a toroid as a one turn winding. The signaling is like HPIL in that each channel transmits in one direction. It is like the IBM PC keyboard in that one start bit plus eight data bits are transmitted and the receiver is a 9 bit shift register, when the start bit reaches the ninth flip-flop, the output interrupts a microprocessor to read the data bits. But where the PC keyboard has a separate clock signal, this circuit recovers a receive clock from the serial stream. There is an edge for every data bit because a pulse of one polarity is transmitted for a "1", and the opposite polarity for a "0". This is similar to the "Manchester encoding" used in Ethernet where the goal is to achieve a zero DC level, which is also part of the reason for "eight-to-fourteen" encoding used in compact disk. By using 14 code bits for 8 data bits, they accomplish run length limiting and have leftover bits (or redundant codes) which are selected to bring the DC average to zero, which is important for the IR receiver.
Just thought you'd like to know!
I'm going to try making a switch from your description of the 9810 keyboard. In high school I built a project that worked on the principle of a shorted transformer secondary. It was a length of 1/4" steel rod bent into a large "U". At each end was a coil of about 100 turns of #20 wire. The "U" was attached to the top of a box with the ends pointing down. A short length of aluminum tube (I used the can of a small electrolytic capacitor) circled the steel rod, free to slide from one side to the other. A motor driven switch energized the coils alternately with about 24 VAC. When the coil under the aluminum tube was energized, the tube would fly to the other side. Another electromagnet project I have plans for, but haven't built, although I have seen one in operation, has an solenoid-type electromagnet mounted on a horizontal support, the coil vertically oriented so the pole is looking down on an open space. A photocell is mounted on a vertical support which is holding the horizontal support. The photocell controls the current to the electromagnet so that the current is in direct proportion to the light hitting the photocell. There are two control pots: one controls the overall gain, and the other controls the response time, from the photocell to the coil. You place a light source so it illuminates the photocell with light passing inder the electromagnet. Now that it's all built, you hold a light steel object, supporting it from the bottom, under the coil so that it is partially blocking the light to the photocell. The perfect object is the tiny globe that comes with a pencil sharpener. Then you adjust the gain till the electromagnet lifts the object, which reduces the light on the photocell, which lowers the drive to the electromagnet. By adjusting the controls you can make the object float. Then you can set it spinning and it will spin for quite a while. I imagine eddy current are what finally slows it down, since it is moving in a magnetic field. With the gain control you can move it up and down a little. With the delay control you can make it bounce up and down a little.