The following are examples of home made solid state relays that could be used to replace mechanical types in many model railroad circuits.
These Relays would be ideal for applications where many relays are needed and the load current requirements are low. Due to their small size a large number of relays could be mounted on a single printed circuit board.
The relays are based on a 4N33 Optoisolator package. This device has a Darlington output transistor and as the first figure shows it can be used alone for low power applications of up to 30Ma. The second figure shows a version for higher current loads of up to 250Ma.
The circuits shown are designed to so that with a controlling current of 10 Milliamps flowing through the LED the voltage drop across the output will be less than 0.3 volts at the circuits rated current.
The current required to operate these relays is significantly less than for mechanical relays types. Also these devices may be less expensive than mechanical relays in many instances as Optoisolators often cost less than a dollar each.
The control and load supply voltages for the examples is 12 Volts, other voltages can be used with a corresponding adjustment to resistors in the circuit. For example if the voltages are doubled then the value of the 1K and/or 470 Ohm resistors should also be doubled in order to keep the currents through the LED and base of Q1 at the same level.
If the 2N3906 transistor is replaced with Darlington type, larger currents can be handled, 1 Amp and more, but the voltage drop across the relay increases and the cost of each unit rises. The diode D1, at the output of FIGURE 2 is a 1N4001. This is an anti-ringing diode and is only really needed if there is an inductive load such as a solenoid being controlled.
The next diagram shows an inexpensive method of building multiple medium current relays.
The circuit uses a ULN2003 - High voltage/High Current Peripheral Driver IC. This device can handle loads of 500 milliamps.
The IC is designed to have TTL and CMOS inputs of between 5 and 15 volts can be controlled by any clean input voltage.
Two methods of controlling the ULN2003 are shown on the diagram. Method "A" is preferable and resistor R1 should be sized to pass about 1 milliamp.
Method "B" is OK but requires an extra diode to compensate for the voltage drop across the optoisolator transistor when it is conducting. If this diode was not used the transistor in the ULN2003 would not be able to turn off fully. Resistor R2 would also be sized to pass about 1 milliamp and the voltage drop across the drivers internal resistor will have to be taken into account.
As the peripheral drivers in the ULN2003 require only a small control current optoisolators with non darlington transistor outputs such as the 4N35 can be used.
There are other devices that are similar to the ULN2003, such as the ULN2803, which has eight drivers in an eighteen pin package. Other devices have higher or lower ratings and/or lack the protection diodes built into the ULN2003 package.
One advantage of Solid State relays is that it has no moving parts and are therefore very fast. This can be very helpful for loads that are switched often as relay noise and wear is eliminated.
Two disadvantages of this type of relay are that three terminals are required for the output side of the higher current version and a voltage drop across the output transistor. In most uses the voltage drop will not affect the circuit load.
Optoisolators also are available in other output configurations and number of units per package. Consult a suppliers parts catalogue for details. Don't hesitate to experiment with these devices you might find an inexpensive solution to a complicated relay problem.
Warning It should be noted that these circuits, unlike mechanical relays are polarity sensitive and will not handle large current surges that might not otherwise affect mechanical relays. Therefore care should be taken before using them in certain circumstances.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.