diff --git a/code/espurna/gpio.h b/code/espurna/gpio.h
index 4fe04b0c..0fb01684 100644
--- a/code/espurna/gpio.h
+++ b/code/espurna/gpio.h
@@ -18,7 +18,7 @@ constexpr const size_t GpioPins = 17;
 // real hardware pin
 class GpioPin final : virtual public BasePin {
     public:
-        GpioPin(unsigned char pin);
+        explicit GpioPin(unsigned char pin);
 
         void pinMode(int8_t mode);
         void digitalWrite(int8_t val);
diff --git a/code/espurna/libs/BasePin.h b/code/espurna/libs/BasePin.h
index 593b5960..cc1814d6 100644
--- a/code/espurna/libs/BasePin.h
+++ b/code/espurna/libs/BasePin.h
@@ -13,7 +13,7 @@ Copyright (C) 2020 by Maxim Prokhorov <prokhorov dot max at outlook dot com>
 
 // base interface for generic pin handler. 
 struct BasePin {
-    BasePin(unsigned char pin) :
+    explicit BasePin(unsigned char pin) :
         pin(pin)
     {}
 
diff --git a/code/espurna/mcp23s08.h b/code/espurna/mcp23s08.h
index 3fede14c..b32e3de8 100644
--- a/code/espurna/mcp23s08.h
+++ b/code/espurna/mcp23s08.h
@@ -26,7 +26,7 @@ constexpr size_t McpGpioPins = 8;
 // real hardware pin
 class McpGpioPin final : public BasePin {
     public:
-        McpGpioPin(unsigned char pin);
+        explicit McpGpioPin(unsigned char pin);
 
         void pinMode(int8_t mode);
         void digitalWrite(int8_t val);
diff --git a/code/espurna/telnet.cpp b/code/espurna/telnet.cpp
index dd3cf7a7..ef08d493 100644
--- a/code/espurna/telnet.cpp
+++ b/code/espurna/telnet.cpp
@@ -34,7 +34,7 @@ struct AsyncBufferedClient {
         constexpr static const size_t BUFFERS_MAX = 5;
         using buffer_t = std::vector<uint8_t>;
 
-        AsyncBufferedClient(AsyncClient* client);
+        explicit AsyncBufferedClient(AsyncClient* client);
 
         size_t write(char c);
         size_t write(const char* data, size_t size=0);