This is not a good design for an (or is it 'a') H-Bridge circuit but it's simple and will work well for low current loads.
The diodes in the base circuit of the PNP transistors might be needed to allow them to cut-off. The resistor in the base circuit depends on the gain of the transistors. A resistor from B+ to the top of the diodes may be needed as well.
The circuit as shown, was tested with a 10 volt supply driving a Tortoise® switch machine with series direction indictating LEDs.
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.
28 January, 2011