The circuit on this page is designed to produce a timed throttle output for demonstrating on board sound systems in a large scale locomotive.
When a push button is activated the output will rise from 0 to 15 volts in 5 seconds. The voltage will then hold for 5 seconds. The voltage will then return to 0 in about 10 seconds.
When S1 is pushed the 555 timer will start and run for 10 seconds.
For the first 5 seconds the 10 uF capacitor at the PLUS input of the LM 358 or LM324 OPAMP will charge until it reaches the voltage at pin 5 of the 555 timer chip plus 0.7 volts
The voltage across the capacitor will now be steady for 5 seconds.
When the timer shuts off the voltage across the capacitor will decay to 0 over a period of 10 seconds, 5 seconds if the optional diode and resistor are used.
The output of the OPAMP drives a conventional transistor throttle output section.
The 500K ohm variable resistors are used to adjust the charging rate so that the capacitor at the PLUS input of the OPAMP reaches it maximum voltage (2/3 of the supply - plus 0.7 volts) in 1/2 of the timers output duration. This control could be replaced by a fixed resistor once the required value is determined.
The circuit is designed to produce an output of15 Volts when operated from a well filtered or regulated 20 Volt power supply only. If voltage adjustment is required the value of the 13K ohm resistor can be increased to give a lower output voltage or it can be decreased to give a higher output voltage.
This circuit has no automatic current limiting or overload protection. With a proper supply the circuit will deliver about 2 amps.
A heat sink may be needed for the output transistor.
The parts values shown on the schematic drawing are calculated to give the desired times and output voltages. Due to value tolerances of all of the parts and the leakage currents of the capacitors some adjustment may be required to achieve the desired results.
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.
15 February, 2009