“I wish to assemble a bargraph display to indicate approximately 25volts dc.How many LM3914 I should cascade? How to calculate resistor value which is grounded from the junction of Pin 6 & 7 (Rhi &Ref.out)? Assuming i need to cascade 3 LM3914s,what is the method to cascade these ics? The display need not be precise or very bright. I hope you can help me out.”
“I shall be very grateful. Can we do it in such a way that the first LM3914 displays 1-10 v & second 11-20v & the third 21-30v?”
Thank you again – Mail from AD
Reply from delabs
Have a look at the application note which has many examples. Understanding this will help you make your own solution.
20-Segment Meter with Mode Switch
Cascading is required if you want more resolution. Example for measuring 20V, you want to show in 1V per LED steps. It can withstand +/- 35 Volts input upper limit.
The inbuilt reference is 1.2V use it as shown in the Typical Application. This also helps in setting the brightness of LED display. You may need to use external attenuators, buffers or precision rectifiers to measure AC voltages or High Level DC voltages.
Recently, IC’s have been introduced that considerably simplify the task of driving a LED array with analog signals. Examples of those include National Semiconductor’s LM3914 and LM3915 LED Dot/Bar Display Drivers. Those extremely versatile devices have a reference, a voltage divider, and ten comparators all on one chip. Besides the LED’s, only a few resistors and a capacitor are required to complete the display circuit. Either a bar or dot display (only one LED on at a time) is possible. The on-chip voltage reference is fully regulated, remaining constant while the power supply feeding the IC can be anywhere between 3 volts and 25 volts !
The LM3914 is a linearly scaled device that’s intended for use in LED voltmeters, with the number of lit LED’s being directly proportional to the input voltage (pin 5). The LM3915 is a logarithmically-scaled device that’s intended for use in power meters, and spans a range of
0-30 dB in ten 3-dB steps. Finally, the LM3916 is a semi-logarithmically-scaled device that’s intended for use in volume-unit (VU) meters.