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Questions about Circuit for Project That Will Be Sent to the ISS

swankybarbecue4

I am designing a circuit for a project that will be sent to the ISS; therefore, I want the circuit to be very reliable and efficient since the total current draw at any one time cannot exceed 500ma, I am hobbyist and had a few questions, so I figured it would be best to ask the experts.

 

A little background on the project:

 

The project will test the affect of reduced gravity on the Alzheimer's beta amyloid peptide. If the project results go our way, then this could lead the first treatment for Alzheimer's, which will be fantastic since it such a horrible disease. So we will have to send those affected to space, which would be super cool!

 

(My schematic is attached)

 

Question 1: Transistors

A) I want to ensure a long life span for each transistor. I have added a 1K resistor to the base of each transistor.

     i) Is this enough resistance?

     ii) How do I find the appropriate resistance for any application?

B) I have also read from this post that if you ad a zener diode as shown in the diagram below it can also benefit the protection of the transistor.

     i) Is this true?

     ii)What does the zener diode do?

C) When I use an NTE123AP transistor (the NTE version of a 3904) to switch my 3.0V vibration motors on and off, they turn off and on fine, but when I use one of the transistors to turn my laser diode off and on, the laser is dim.

     i) What causes this?

     i) Is this due the transistor's drop-out voltage?

D) Germanium diodes

     i) What are germanium diodes?

     ii) Would they better suit my application?

     iii) What are advantages/disadvantages of silicon and germanium when compared?

image

Question 2: Relays

A) To solve the dimming issue I had when powering my laser diode, I decided to have the transistors turn relays off and on, which in turn would switch my laser off and on (as shown in my schematic).

      i) Am I driving these relays in the most efficient manner?

      ii) Is it necessary to use a relay for the vibration motors?

 

Question 3: Additional Suggestions 

A) Any other tips, suggestions of how things differently, or rules of thumb that you think I should know about? I would greatly appreciate the feedback!

 

 

I really appreciate any help!

 

Cheers,

 

 

Merrick

Attachments:
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  • Hi Merrick,

     

    You will get better results in your switching circuits if you use the transistor in this configuration. It will act more like a switch.

     

    image

    Click on the image for a larger view. I have also suggested a N Channel MOSFET alternative to the transistor which should save you some wasted power.

    If you use the MOSFETs I do not believe that you will need relays which should greatly reduce the weight and power consumption. Depending on the current needed by the motors you may need to upgrade my suggested MOSFETS as they are capable of only 200 mA continuous. I have shown the schematic with standard silicon general purpose diodes. If you want a slightly lower junction voltage you could use a schottky diode like a 1N5819. Germanium is not used in these applications to my knowledge. I also believe that this configuration of transistor and load will solve your dimming problem with the laser without need for a relay.

     

    John