Now for the complete schematics. This schematic is Copyright by myself 2011, but I grant permission for anyone to use it for personal purposes. If you'd rather not build it yourself, I am selling the blank PCB to help offset my design costs and keep my own per boards cost down. I don't have a high markup, the intent is to keep the price down to help out hobbyists and experimenters.
The final PCB measures 3"x4.5" and has four plated through mounting holes spaced 2.5"x4" apart for #6 screws. In my mounting for the train set, I used some cheap plastic spacers and wood screws to mount them to the 2x4's of the train layout.
Previous Section: detailed descriptions
Assembly Options
Most of the assembly options ar compatible with each other with the exception of only one output MOSFET/Transistor per stage is permitted, and a high voltage output with voltage regulator can't be combined having a low voltage power supply input for the timing and logic stage, Even having the extra diodes on the output stage used for transistors is safe to combine with MOSFET's since they will simply add extra protection.
When you don't need the flexibility of using jumpers as options, I encourage you to replace the headers with a wire jumper. The designs allow for jumpers for when flexibility is required. This is also why some jumpers are in pairs so that either two 1x3 or one 2x3 header can be used as well.
For capacitors C1, C16, C17, and C18 only install as much as needed. Using all four at 1000uF each is only needed if you have a weak power supply and high current devices. having a single 100uF capacitor works for good quality power supplies. When using high output voltage, at least one capacitor in each section should be installed after cutting the runs on the top layer (marked by the X's) and installing either a 7805 or a 7812 in the holes near the top X in the picture if you want to power the low voltage from the high voltage. It's recommended to also add a diode between the low voltage and the high voltage to help drain the low voltage capacitors faster as a fail-safe.
Connector J1 is there for the convenience or using a ribbon cable. I expect most people won't be using that and and be omitted. Input connectors J2 & J3, output connectors J6 & J8, and power connectors J4, J5 & J7 have extra large pads and larger then needed holes to allow direct soldering or wires people people trying to save a little. Also, many people will only need one of the three power connectors, the additional connectors are to make it easier to choose where power comes in, daisy chaining boards, and the high voltage output stage option.
For two wire only assembly, input connectors J2 & J3 can be two position, leaving the third pin unused. When two wire with a single common voltage is used that the main control signal goes above or below, you could make J2 a 3 position connector, and then solder a wire from J2-3 to J3-1 and install the common jumper JP3. When the common voltage changes or is not the same, do NOT jumper JP3 since that shorts J2-2 & J3-2 together.
Normally jumpers JP1 & JP2 need to be in the same position as do JP4 & JP5 to select two wire or three wire operation. Jumper 1-2 for two wire operation and 2-3 for three wire inputs. Each half of the board though does not need to be the same, but be careful of the JP3 common jumper that shorts the two common inputs together, specially since some two wire modes change the voltage on this input.
In the Timing circuit, JP6, JP7, JP11, JP13 determine if the timers are edge or level triggered, using edge triggered allows for safe control of coil/relay style switches, Level triggered should be used only for devices that are safe to be on constantly, such as stall motors (Tortoise) and lights. If you will be using only level triggering, then the pulse capacitors C2, C4, C8, C10 can be omitted along with R7, R10, R13, R14
Paired output and timer resetting is controlled by JP8, JP9, JP10 & JP2. When in position 1-2, an input will clear out the timer on the other output for that pair to prevent turning on both sides of the output at the same time. This is highly recommended (almost required) for good three wire output to snap/coil style switches. But, when you want to use the output independently, you don't want that to happen and connect pins 2-3 together. People experimenting or needing other additional logic can even jumper other signal into pin 2 or wire things differently (such as JP8-2 to JP12-1 to watch a different input).
For Transistor output, D1, D3, D5, and D7 are required when driving relay, coils or anything they might deliver a back voltage. When driving lights or using MOSFET's, these diodes aren't needed.
The resistors R21-R24 are used only with low current two wire output mode they can be omitted in three wire mode. The values can easily be adjusted for two wire mode, but remember that the MOSFET/Transistor used with them will need to draw twice the current in two wire mode.
For High Current output, I recommend using MOSFET's rated for higher voltage and at least 5 times the current for driving typical snap/coil type switches. Having them them needing to draw 3A is common, which is why in my design & tests I show a 20A part. This allows for good snappy action and at the same time no heat sink is required unless the turnout is used a lot. In my tests win an older concor 12V dual coil switch, the switch was getting hotter faster then the MOSFET's were. In general with MOSFET's, the higher the current rating the cooler they will run and often be more expensive, but even many 40A parts are fairly cheap. Some room has been reserved for small heatsinks to be used, but be careful not to let them short out to other parts. Heatsinks may be required for high current constant use.
Timing Options
As stated above, the timing control can be edge or level triggered. When it is level triggered, the timers are used to debounce the input signals and fairly short timing is needed. When edge triggered, the timers determine how long the outputs stay on. I;ve found that 1/5th of a second works well for snap/coil switches and about 8-10 seconds for tortoise switches works well.
There are 4 pairs of parts that control the timing, R8+C3, R15+C12, R11+C6, R16+C15. Each pair controls the timing for one output. Adjusting their values down shortens how long they will be on.
Some approximate sample values are listed below:
| R | C | Timing |
|---|---|---|
| 1M ohm | 0.1uF | 1/10th sec |
| 2M ohm | 0.1uF | 1/5th sec |
| 1M ohm | 10uF | 10 sec |
Other combinations of values can easily be used to get the same timing. These are simply based on RC time constants and measured with parts I had on hand. Since most of the boards for me needed the 1/5th second timing, being able the same type of capacitor for timing and power filtering was handy.
