• Infrared LED

    4 days ago 0 comments

    Many years ago when I was designing an infrared camera slave pointing system the cost of an infrared receiver IC was about $7. The IC was TTL (Transistor Transistor Logic) with three pins. Power, Output and Ground. Then the price increased to about $8 which when I actually purchased this IC in late 2002. I also obtained a copy of the datasheet from the electronics store. However, the printing was low quality and I could not read the pins properly. I must have made a mistake with the pins and partially burned this IC. The IC was receiving the infra red signal well. However, it a weak output drive because I might have partially burned it.

    Nowadays the cost is only about $3. This make this ICs appropriate for cheap circuits.

    First I read the following articles about old fashioned transistor amplifiers that can be used to amplify signals from infrared sensors:

    https://www.instructables.com/id/Transistor-Sensor-Amplifier/

    https://www.instructables.com/id/Recycled-Transistor-Amplifier/

    I designed the circuit with the ZD1953 IC and a simple PNP BJT (Bipolar Junction Transistor) amplifier.

    Two transistors were placed in parallel to improve reliability and increase device life time. However, using a second transistor is not necessary. The circuit can still work well with one transistor.

    From top to bottom the pin layout is:

    1. Output (connected to 10 kohm resistor)

    2. Ground (blue wire)

    3. 5 V Power Supply for TTL (red wire)

    The PNP transistor collector is connected to two LEDs in series with the blue 100 ohm resistor. The voltage potential is:

    VoMax = Vled1 + Vled2 + Vr

    = 2 V + 2 V + 100 ohms * 10 mA = 5 V

    I placed the circuit in a plastic container because I did not want to pay for the box.

  • Cheap Noise Generator

    09/03/2020 at 09:53 0 comments

    This article is about a cheap noise generator that I made.

    I found a cheaper solution to the circuit constructed in this article:

    https://www.instructables.com/id/Random-Number-Generator-2/

    I used an old black box from a very low frequency (VLF) noise generator, disassembled the circuit and used the parts to make a cheap audio noise generator:

    Step 1: Design the Circuit

    I designed the circuit with two simple BJT transistor feedback bias amplifiers:

    The input signal comes from EMI (electromagnetic interference) in a loose 20 cm (in length) insulated wire connected the amplifier input.

    You might be able to increase the circuit gain if you change Rc1 and Rc2 values from 1 kohm to 10 kohms. However, reducing the collector biasing current can also reduce the transistor current gain. It all depends on the transistors that you use. 

    Different transistors have different current gain due to production tolerances. Transistor current gain can also changes with temperature and aging.

    Step 2: Simulations

    I used the old PSpice simulations software that has a quick user interface.


    Transient:

    Frequency:

    You might not be able to obtain such as high bandwidth with general purpose transistors. 

    Step 3: Make the Circuit

    I did not include the RC power supply filters in my circuit to reduce the cost of my circuit because I have high current/low internal resistance power supply.

    You do not need a high power resistor to drive the load. I did not have any other resistors in stock. 

    I also omitted Cb1 capacitor that is only needed for preventing the input for affecting the biasing of the first transistor Q1.

    Step 4: Testing

    I connected the output to USB oscilloscope:

    The noise generator was generating noise even when I was not touching the yellow input wire with my fingers.

    I noticed that the power supply needs to be at least 3 V. When I reduced the power supply to 1.5 V (the minimum supply voltage for bipolar junction transistors), the noise signal magnitude fell to a very small value, because reducing supply voltage also reduces the transistor current gain.

  • Cheap Opamp Power Supply

    03/29/2020 at 12:35 0 comments

    The circuit shown in the paper can provide a maximum current of only 10 mA. There are also limitations in the minimum and maximum voltage output.

    I made a similar circuit many years ago. It stopped working due cracked wires. I completely disassembled the circuit and made from scratch but with the same opamp IC and the same IC socket. I usually use wire wrap. However, the IC socket had short legs because its not a wire wrap socket. Thus I had to user a soldering iron. I also used a 67 kohm potentiometer from an old VCR (video cassette recorder).

    You do not need to user a 14 pin IC socket that I used. You can purchase an 8 pin wire wrap IC socket.

    You can see the circuit operation in this video:

    Designing the Circuit

    I have drawn the circuit in https://easyeda.com online software.

    The circuit below is showing a simple opamp voltage follower configuration with a very high input resistance.

    Rv can be of any potentiometer value from 10 kohms to 1 Megohms. Reducing Rv to just 1 kohm will a reasonable amount of current from power source and will waste power. However, there is still a small amount of power consumed by the operational amplifier IC even when the output current is zero.

    The 4.5 V power source is the minimum that you can supply to a general purpose opamp.

    If the power supply is raised to 15 V and opamp saturation voltage is 3 V, the maximum opamp output voltage will be 12 V.

    The maximum LED current will equal to:

    IledMax = (Vo - Vled) / Rd = (12 V - 2 V) / 1000 ohms

    = 10 V / 1000 ohms = 10 mA

    Ro is used for short circuit protection to prevent damage to the opamp IC.

    The maximum opamp output current at 12 V output will equal to:

    IopampMax = Io + Iled = 12 V / 1000 ohms + 10 mA = 22 mA

    This is twice the maximum opamp output current. However, this occurs only when the output is shorted and Rl is zero.