Hello! I’m trying to control a 5V 1A solenoid with a knock sensor that produces 1V. I’ve been an auto tech for years so my first thought was finding a relay of some sort. The control side of the relay would again be controled by 1V and the switch side would need to be able to handle 5V at 1A. I’ve discovered solid state relays from digikey that seem like they’ll work but there is a lot of options and I want to make sure im not overlooking something. Also, is there an option for another component that could control this circuit? I had an electrical engineer suggest a Mosfet but I was struggling to find one that closes a circuit with just 1V. Do you have any suggestions?
If you’re looking for a relay that switches at 1V then:
Have a look at this module, 1738-1077-ND (DigiKey part number). It employs a NPN transistor - this will start to conduct with a voltage across the base/emitter junction around 0.6V. It needs a 5V supply, and an input of around 1V to energise the relay. My only concern is the R2/R3 values presented in the schematic - these form a potential divider such that with a 1V input the voltage at the base will be in the order of 0.65V - which should be just enough to actuate the relay. If you have problems with this switching reliably then, depending on your ability to rework the board I would first remove the R3 component (the 10k resistor) which will probably bear the marking ‘103’. If you still have a problem then I’d look to reduce the value of R2 (marking ‘472’) either by placing another resistor in parallel or replacing directly - I’d recommend value of around 270 Ohm (‘271’). This assumes that the sensor is capable of sinking a current of a milliamp or more.
However, you mention a knock-sensor - typically a piezo-electric sensor that will generate a signal based upon ‘knock’ from the engine. This is typically a short-duration signal, with a period measured in 10’s or 100’s of milliseconds. For this you’ll need to consider what behaviour you’re looking to deliver. An electro-mechanical relay such as above will have finite switching times (measured in 10’s of milliseconds) and you would probably encounter ‘chatter’ with the relay energising/de-energising as the sensor output oscillates. You further mention that you’re looking to actuate a solenoid - these similarly have finite actuation speeds and it is possible that the signal from the sensor will have abated before the solenoid finally actuates. Some experimentation is required here.
A “proper” solution may need a clearer description of what you are trying to achieve. I’m guessing that you are looking to actuate a solenoid whilst knock-energy is present and release the solenoid in the absence of a signal from the knock sensor?, in which case you need to understand the time period that may be expected between knocks under the condition that knock is present (rather than trying to chase the signal with a relay). If you search the internet for “Knock Shield”, this problem has been examined and a product is available commercially to investigate engine knock; there is a good explanation of the behaviour of an engine knock sensor and the processing necessary to use such a sensor.
Welcome to the Digikey tech forum. You can take a look at Digikey part 264-TLP3406SRHA(TPECT-ND as a potential option.