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Need an Amplifier Circuit

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I'm in need of an amplifier circuit that will amplify a 2.0 VDC. pulse signal to a > 12 VDC pulse signal, so it can be read by an Automation Direct DL06 PLC high speed counter input. I bought some 2n3904 transistors and some 1k, 2k, and 10k resistors hoping I could make them work. It's been a lot of years since I've done anything like this and I need some help designing the circuit. Can anyone help me out? The frequency is going to be around 600 Hz and the input voltage to the high speed counter input needs to be at least 12 VDC.


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Do you have a 12 V source to power the circuit? If so, the circuit is
1 - 2N3904
1 - 1.0 K
1 - 10K


dr pepper

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2n3904 emmiter to ground, base via 10k resistor to 2v signal, collector to 1k resistor then other side of 1k to +12v, signal output comes off the collector.
You might need to lower the 10k to 4k7, or 2k2.

Les Jones

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One point that may be important is that the single transistor circuit will invert the signal. This is probably not imprtant as the signal is going to a PLC so it would just need the code changing to take this into account. If it does matter 2 transistors would be required. (Or use a comparitor IC such as an LM393)



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There is a bit more to it.

I looked up the specs for a DL06 DC input:



The DL06 can source 24Vdc at one of it's pins, so I am using that as the power supply. The maximum pull-up resistor (to 24V) that would satisfy the DL06 input requirement is about 2K, so in the low state, the switching transistor Q1 would have to sink ~6mA.

Since you didn't specify what sort of load the sensor can drive, I use a second 2N3904 (Q2) primarily to keep the input impedance high (so as not to load the sensor), and to make it so that there is no net inversion between the sensor and the input:


The unknown source resistance Rs could be anything from zero to ~50KΩ and the circuit would still work...

The trip point for V(sensor) is about 0.7V. The current from the sensor is shown as I(R1). The voltage to the PLC V(plcin) satisfies the spec, and is >12V.

ps: One other potential problem occurred to me after the fact. Is the return for the PLC's 24V power supply connected to the common pin for all of the inputs? if not, you might need a separate, external, floating 24Vdc power supply to power the interface circuit?
Alternatively, you could tie the opt0-isolated "common" pin to the the PLC's Gnd, but that would undo the opto-isolation on the inputs...
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